feat: add Sixel graphics backend with protocol auto-detection

Introduce backend dispatch architecture so the package supports multiple
terminal graphics protocols.  Existing Kitty protocol code is extracted
into a `kitty` backend; a new `sixel` backend enables ~20 additional
terminals (foot, Konsole, xterm, mlterm, mintty, VS Code Terminal).

Key changes:
- Alist-based backend dispatch (`kitty-gfx--backends`, `kitty-gfx--backend-fn`)
- Kitty backend: detect/prepare/place/delete/cleanup extracted from inline code
- Sixel backend: ImageMagick-based encoding, temp-file cache, DCS emission
- Auto-detection (`kitty-gfx--detect-protocol`): tries Kitty first, then Sixel
- `kitty-gfx-preferred-protocol` defcustom for user override (auto/kitty/sixel)
- Mode-line indicator shows active backend: [K], [S], or [?]
- Fix obsolete `when-let` → `when-let*` (zero byte-compile warnings)

All integration points (org, image-mode, shr, dired, doc-view) dispatch
through the backend interface — no mode-specific changes needed.

0 files changed, 0 insertions, 0 deletions