Optimize iTerm2 palette encoding for faster GUI updates

[mesolimbo]

Motivation

• Reduce latency for the iTerm2 GUI path by avoiding expensive RGB->JPEG work when a palette fits into 256 colors, improving responsiveness on macOS without changing UI appearance.

Description

• Add a cached palette byte buffer _ITERM2_PALETTE_BYTES and a helper _get_iterm2_palette_bytes() to produce a 256-color palette suitable for paletted PNGs in gui/sixel.py.
• Update pixels_to_iterm2() to prefer paletted PNG encoding (1 byte per pixel) when the color table fits in 256 entries and to fall back to the existing JPEG path for large palettes.
• Invalidate the new palette cache when dynamic image colors change by updating register_image_colors() and invalidate_iterm2_color_cache() to clear _ITERM2_PALETTE_BYTES as well as the color table.
• Adjust documentation/comments to reflect the new paletted-PNG optimization and keep behavior unchanged for non-iTerm2 or PIL-missing environments.

Testing

• No automated tests were run as part of this change.

---

Codex Task

[mesolimbo]

Merged #22 instead