feat: add Sixel graphics backend with protocol auto-detection

- Introduce alist-based backend dispatch (kitty/sixel)
- Add Sixel encoding via ImageMagick with temp-file cache
- Auto-detect protocol: try Kitty first, then Sixel
- Add kitty-gfx-preferred-protocol defcustom
- Sixel erase via space-overwriting (stateless protocol)
- Cleanup temp files on kill-emacs
- Update README and AGENTS.md for multi-protocol support
- Bump version to 0.3.0

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+# PLAN: Sixel Backend Expansion — terminal-graphics.el
+
+## Meta
+- **Date**: 2026-03-20
+- **Mode**: SCOPE EXPANSION
+- **Version**: 0.2.1 → 0.3.0
+- **Estimated Effort**: 28-41 hours (~3-5 days)
+- **LOC Delta**: +300-400 lines (1948 → ~2300)
+- **Risk**: MEDIUM (terminal quirks)
+- **Confidence**: HIGH (clean abstraction perimeter)
+
+---
+
+## Motivation
+
+kitty-graphics.el is the first-ever native Emacs Lisp package for terminal
+image display. Currently supports Kitty/WezTerm/Ghostty (~3 terminals).
+Sixel support unlocks ~20 additional terminals including foot, Konsole,
+xterm, mlterm, mintty, VS Code Terminal, **and tmux**.
+
+tmux is the killer use case — it's the only way to get images in tmux
+from Emacs, since Kitty protocol doesn't work through tmux.
+
+## Key Architecture Decisions
+
+### 1. Backend Dispatch: Alist (Decision A)
+
+A backend is an alist of functions:
+```elisp
+((prepare . kitty-gfx--kitty-prepare)
+ (place   . kitty-gfx--kitty-place)
+ (delete  . kitty-gfx--kitty-delete)
+ (cleanup . kitty-gfx--kitty-cleanup))
+```
+
+`kitty-gfx--backends` is an alist of `(SYMBOL . BACKEND-ALIST)`.
+`kitty-gfx--active-backend` holds the active key ('kitty or 'sixel).
+Dispatch: `(funcall (alist-get 'place (alist-get backend kitty-gfx--backends)) ...)`
+
+**Why**: ~2h more than if/else, but every future protocol (iTerm2, etc.)
+is a 5-line alist entry instead of branches everywhere.
+
+### 2. Sixel Cache: Temp-Files (Decision B)
+
+Sixel-encoded data is stored in temp-files under `/tmp/kitty-gfx-sixel-*.six`.
+Cache lookup: file-exists-p → insert-file-contents → send-string-to-terminal.
+
+**Why**: Eliminates GC pressure from multi-100KB Emacs strings. Disk I/O
+is negligible vs. the ImageMagick shell-out that already happens.
+Cache-size can stay at 64 entries. Cleanup via kill-emacs-hook.
+
+### 3. Refresh Strategy: Dirty-Flag (Decision A)
+
+Sixel overlays are only re-emitted when `last-row`/`last-col` has changed
+OR an explicit dirty flag is set (resize, buffer-switch). Standing images
+on a static screen = zero cost.
+
+**Why**: Minimal-invasive. One bool per overlay. The existing position
+tracking provides the dirty signal for free.
+
+### 4. Encoding: Synchronous with Debounce (Decision B)
+
+Window resize triggers synchronous re-encode with 500ms debounce for
+Sixel backend. User sees 1-2s freeze for 5+ visible images on resize.
+Accepted tradeoff — resize is infrequent.
+
+**Why**: Async encoding (make-process + sentinel) adds race conditions
+and ~6h complexity for a rare interaction. Ship first, optimize if
+anyone complains.
+
+---
+
+## Protocol Comparison
+
+```
+                   KITTY PROTOCOL                    SIXEL PROTOCOL
+  Statefulness:    STATEFUL (transmit once,          STATELESS (emit all data
+                    place many times)                  every time)
+  Image Storage:   TERMINAL-SIDE                     CLIENT-SIDE (temp-files)
+  Placement:       NAMED (PID) — atomic replace      ANONYMOUS — erase + re-emit
+  Delete:          EXPLICIT (APC a=d)                 IMPLICIT (overwrite spaces)
+  Encoding:        BASE64 PNG (lossless)              6-bit palette + RLE
+  Wire Format:     APC (\e_G...\e\\)                  DCS (ESC P q ... ESC \)
+  Max Colors:      Full RGBA (truecolor + alpha)      256 colors (no alpha)
+```
+
+---
+
+## System Architecture — Target State
+
+```
+  ┌─────────────────────────────────────────────────────────────┐
+  │                    INTEGRATION LAYER                         │
+  │  org-mode │ image-mode │ shr │ doc-view │ dired │ eshell   │
+  │  (20+ :around advice functions — ALL UNCHANGED)             │
+  │           ALL call: kitty-gfx-display-image                 │
+  └──────────────────────────┬──────────────────────────────────┘
+                             │
+  ┌──────────────────────────▼──────────────────────────────────┐
+  │                    ORCHESTRATION LAYER                       │
+  │  kitty-gfx-display-image                                    │
+  │    1. convert-to-png  (ImageMagick — shared)                │
+  │    2. compute-cell-dims (shared)                            │
+  │    3. DISPATCH to active backend:                           │
+  │       (funcall (kitty-gfx--backend-fn 'prepare) ...)        │
+  │    4. make-overlay (shared)                                 │
+  │    5. schedule-refresh                                      │
+  └──────────────────────────┬──────────────────────────────────┘
+                             │
+  ┌──────────────────────────▼──────────────────────────────────┐
+  │                    BACKEND DISPATCH                          │
+  │                                                             │
+  │  kitty-gfx--active-backend ──▶ 'kitty / 'sixel             │
+  │                                                             │
+  │  ┌─────────────────┐    ┌──────────────────────────┐       │
+  │  │  KITTY BACKEND   │    │  SIXEL BACKEND            │       │
+  │  │                  │    │                           │       │
+  │  │  prepare:        │    │  prepare:                 │       │
+  │  │   transmit APC   │    │   png→sixel encode        │       │
+  │  │   cache: id only │    │   cache: temp-file path   │       │
+  │  │                  │    │                           │       │
+  │  │  place:          │    │  place:                   │       │
+  │  │   APC a=p (50B)  │    │   read tmpfile → DCS Pq   │       │
+  │  │                  │    │   (50-500KB)              │       │
+  │  │                  │    │                           │       │
+  │  │  delete:         │    │  delete:                  │       │
+  │  │   APC a=d (30B)  │    │   overwrite with spaces   │       │
+  │  │                  │    │                           │       │
+  │  │  cleanup:        │    │  cleanup:                 │       │
+  │  │   delete-by-id   │    │   delete temp-files       │       │
+  │  └─────────────────┘    └──────────────────────────┘       │
+  └─────────────────────────────────────────────────────────────┘
+```
+
+---
+
+## Protocol-Specific Functions — Refactor Map
+
+Only 6 functions emit terminal-specific escape sequences today.
+These become backend dispatch points:
+
+| Current Function              | Backend Method | Kitty                | Sixel                     |
+|-------------------------------|---------------|----------------------|---------------------------|
+| `kitty-gfx--transmit-image`   | `prepare`     | APC a=t chunked      | magick → sixel tmpfile    |
+| `kitty-gfx--place-image`      | `place`       | APC a=p (50 bytes)   | read tmpfile → DCS emit   |
+| `kitty-gfx--delete-placement` | `delete`      | APC a=d,d=i          | overwrite with spaces     |
+| `kitty-gfx--delete-by-id`     | `cleanup`     | APC a=d,d=I          | delete temp-file          |
+| `kitty-gfx--delete-all-images`| `cleanup-all` | APC a=d,d=A          | delete all temp-files     |
+| `kitty-gfx--supported-p`      | (detection)   | KITTY_PID/TERM check | DA1 query + env check     |
+
+Everything else (~70% of codebase) is protocol-agnostic.
+
+---
+
+## Performance Model
+
+```
+  OPERATION                  | KITTY            | SIXEL              | RATIO
+  ---------------------------|------------------|--------------------|--------
+  First display (cold)       | ~200ms           | ~300ms             | 1.5x
+  Placement (warm cache)     | ~0.1ms (50B)     | ~5-50ms (50-500KB) | 50-500x
+  Refresh (no change)        | ~0ms (skip)      | ~0ms (dirty-flag)  | 1x
+  Refresh (position change)  | ~0.1ms           | ~10-100ms          | 100-1000x
+  Window resize (5 images)   | ~0.5ms           | ~1500ms            | 3000x
+  Delete                     | ~0.1ms           | ~1-5ms             | 10-50x
+```
+
+Dirty-flag eliminates steady-state cost. Resize is debounced (500ms).
+
+---
+
+## Error & Rescue Registry
+
+| Method                    | Error                 | Rescued | Action                | User Sees           |
+|---------------------------|-----------------------|---------|-----------------------|---------------------|
+| detect-protocol (DA1)     | timeout               | Y       | env-var fallback      | Transparent         |
+| detect-protocol (DA1)     | parse-error           | Y       | env-var fallback      | Transparent         |
+| sixel-encode              | missing ImageMagick   | Y       | user-error            | "Install ImageMagick"|
+| sixel-encode              | process-error         | Y       | skip image, log       | Image missing       |
+| sixel-encode              | empty output          | Y       | skip image, log       | Image missing       |
+| sixel-place (DCS emit)    | io-error              | Y       | ignore-errors         | Image missing       |
+| sixel-cache-put           | disk full             | Y       | condition-case        | Fallback in-memory  |
+| backend-dispatch          | missing backend key   | Y       | error with message    | Clear error msg     |
+| sixel-delete (erase)      | position stale        | PARTIAL | sync-begin/end        | Brief flicker       |
+
+CRITICAL GAPS: 0
+
+---
+
+## Failure Modes Registry
+
+| Codepath         | Failure Mode        | Rescued? | Test? | User Sees?  | Logged? |
+|------------------|---------------------|----------|-------|-------------|---------|
+| sixel-encode     | magick not found    | Y        | #2    | user-error  | Y       |
+| sixel-encode     | corrupt PNG input   | Y        | —     | skip image  | Y       |
+| sixel-encode     | huge output (>1MB)  | Y        | #16   | downscaled  | Y       |
+| sixel-place      | terminal overflow   | PARTIAL  | #6    | slow/hang   | Y       |
+| sixel-cache      | disk full           | Y        | —     | fallback    | Y       |
+| detect-protocol  | DA1 timeout         | Y        | #5    | fallback    | Y       |
+| detect-protocol  | terminal lies       | N        | —     | wrong proto | N       |
+| refresh (sixel)  | rapid scroll 5 imgs | Y (flag) | #6    | skip re-emit| Y       |
+| resize (sixel)   | 5 imgs re-encode    | Y (dbnc) | #7    | 1-2s freeze | Y       |
+
+---
+
+## Rollout Phases
+
+### Phase 1: Backend Refactor (in-place)
+- Introduce alist dispatch
+- Extract existing Kitty code into 'kitty backend
+- NO new functionality, only restructuring
+- Byte-compile + load-test
+- Commit: "refactor: extract protocol backend dispatch"
+
+### Phase 2: Sixel Backend
+- kitty-gfx--sixel-encode, -place, -delete, -cleanup
+- Temp-file cache system
+- Dirty-flag in refresh cycle
+- Commit: "feat: add Sixel graphics protocol backend"
+
+### Phase 3: Auto-Detection
+- DA1 query + env-var fallback chain
+- kitty-gfx-preferred-protocol defcustom ('auto / 'kitty / 'sixel)
+- Commit: "feat: auto-detect terminal graphics protocol"
+
+### Phase 4: Polish
+- Repo rename: kitty-graphics → terminal-graphics.el
+- README update
+- Version bump: 0.3.0
+- MELPA recipe submission
+
+---
+
+## Test Matrix
+
+| # | What                          | Type   | Spec                                    |
+|---|-------------------------------|--------|-----------------------------------------|
+| 1 | Sixel encode happy path       | Manual | PNG in org-buffer, foot terminal        |
+| 2 | Sixel encode failure          | Manual | Remove ImageMagick, attempt display     |
+| 3 | Backend auto-detect Kitty     | Manual | In Kitty: mode on → backend = 'kitty   |
+| 4 | Backend auto-detect Sixel     | Manual | In foot: mode on → backend = 'sixel    |
+| 5 | Fallback when DA1 fails       | Manual | Set TERM_PROGRAM manually, block stdin  |
+| 6 | Scroll performance (5 images) | Manual | Org file with 5 images, rapid scroll    |
+| 7 | Window resize                 | Manual | Display image, C-x 3 split → re-encode |
+| 8 | Cache hit                     | Manual | Toggle images off/on → faster 2nd time  |
+| 9 | Cache cleanup on kill-emacs   | Manual | Exit Emacs, check /tmp → files gone     |
+| 10| tmux + Sixel                  | Manual | tmux (--enable-sixel) + foot            |
+| 11| Kitty regression              | Manual | Kitty: +/-/0 zoom still works           |
+| 12| Sixel image-mode zoom         | Manual | foot: +/-/0 zoom → re-encodes          |
+| 13| doc-view in Sixel             | Manual | foot: open PDF → pages render           |
+| 14| shr/eww in Sixel              | Manual | foot: eww with images                   |
+| 15| defcustom override            | Manual | Force 'sixel in Kitty → uses Sixel      |
+| 16| Large image DoS protection    | Manual | 4K image → capped, no terminal hang     |
+
+---
+
+## NOT in Scope
+
+| Item                         | Rationale                                        |
+|------------------------------|--------------------------------------------------|
+| iTerm2 Protocol Backend      | Trivial after Sixel (~2h), separate PR           |
+| tmux Kitty Passthrough       | Separate feature, separate PR                    |
+| Async Sixel Encoding         | Only if users complain about resize freeze       |
+| OSC 66 Text Sizing Ship      | Already designed in AGENTS.md, separate feature  |
+| GIF/Animation Support        | Out of scope for this iteration                  |
+| In-Emacs Sixel Encoder       | Would eliminate ImageMagick dep, ~40h, not now   |
+| comint-mime Integration      | Issue #5, separate feature                       |
+
+---
+
+## Delight Opportunities (post-ship)
+
+1. **tmux Kitty passthrough** — DCS wrapper, ~1h after backend abstraction
+2. **Auto-detect with modeline indicator** — `[K]` or `[S]` in lighter
+3. **kitty-gfx-preferred-protocol defcustom** — user override for power users
+4. **Sixel color palette size defcustom** — 256 vs 1024, speed vs quality
+5. **iTerm2 backend** — ~2h, covers macOS iTerm2 users
+
+---
+
+## Naming Strategy
+
+- Emacs symbol prefix: keep `kitty-gfx-` (breaking change otherwise)
+- Repository name: rename `kitty-graphics` → `terminal-graphics.el`
+- GitHub rename preserves all stars, forks, issues, redirects old URLs
+- MELPA package name: `terminal-graphics` (or keep `kitty-graphics`)
+- Explain in Commentary section why prefix differs from repo name
+
+---
+
+## GitHub Rename Notes
+
+- Stars, forks, issues ALL preserved on rename
+- Old URLs auto-redirect to new name
+- git clone/fetch/push from old URL continues to work
+- CAVEAT: "do not reuse the original name" — if you create a NEW repo
+  called `kitty-graphics` after renaming, redirects break
+- Recommendation: simple rename, no new-repo trick needed