fix: Sixel backend parity improvements

Close gaps identified in Sixel vs Kitty audit:

- image-mode: mode-name now shows Image[Sixel] or Image[Kitty]
  based on active backend (was hardcoded to Image[Kitty])
- LaTeX preview: docstring updated to reference kitty-graphics
  package name, not Kitty protocol specifically
- sixel-cleanup-all: now erases visible Sixel images from terminal
  cells before cleaning disk/memory state (prevents pixel artifacts
  on mode disable)
- sixel-place: LRU eviction for .six temp files, capped at
  kitty-gfx-cache-size (default 64), oldest evicted on overflow

Remaining by-design differences (protocol limitations, not bugs):
- Sixel re-emits raw pixel data on scroll (no placement IDs)
- Sixel erases by overwriting with spaces (no protocol-level delete)
- Sixel detection is env-var based (no DA1 query yet)

0 files changed, 0 insertions, 0 deletions