terminal/kitty: accept first-frame GIF images via Wuffs

[deblasis]

Summary

Extend the Kitty graphics image decoder to handle GIF payloads, rendering the first frame only. Animation requires a separate design pass (renderer-side clock, disposal compositing, repaint hooks); this PR establishes the still-image plumbing so emitters using iTerm2 OSC 1337 File= can ship GIF logos and screenshots.

Seventh PR in the iTerm2 OSC 1337 series (after #375 #376 #377 #378 #379 #380). Closes the image-format gap on the iTerm2 synth path: PNG + JPEG + GIF.

Architecture

Mirrors PNG/JPEG plumbing end-to-end:

• pkg/wuffs/src/c.zig enables WUFFS_CONFIG__MODULE__GIF (and the WUFFS_CONFIG__MODULE__LZW module the GIF decoder depends on). The build flag flows through to the Wuffs amalgamation via the existing defines -> flags pipeline in pkg/wuffs/build.zig.
• New pkg/wuffs/src/gif.zig wrapper mirrors jpeg.zig with two GIF-specific additions:
  • The decode_frame_config step that GIF requires before decode_frame (PNG/JPEG short-circuit because they are single-frame).
  • A @memset on the destination buffer so sub-canvas frames keep transparent pixels rather than carrying debug-allocator poison bytes.
• After decoding the first frame, a second decode_frame_config call detects multi-frame sources and emits a log.debug so dropped frames are visible at --log-level=debug.
• decode_gif sys slot in src/terminal/sys.zig, GhosttySysDecodeGifFn typedef + GHOSTTY_SYS_OPT_DECODE_GIF=4 enum entry in include/ghostty/vt/sys.h, new .gif Format variant + GHOSTTY_KITTY_IMAGE_FORMAT_GIF=6 in the kitty graphics C ABI, init guard + shared-memory size + complete() dispatch arms in src/terminal/kitty/graphics_image.zig, renderer switch arm in src/renderer/image.zig, and iTerm2 OSC 1337 synth sniff for the "GIF8" magic (47 49 46 38) covering both GIF87a and GIF89a.

Tests

• 2 new wuffs unit tests (gif_decode_000000, gif_decode_FFFFFF).
• 2 new C ABI tests (set decode_gif with null clears, set decode_gif installs wrapper) mirroring PNG/JPEG coverage.
• 1 new iTerm2 synth test using a GIF89a header payload.
• Rename of the existing "non-PNG non-JPEG bytes" test to "unknown bytes".
• Full zig build test green on Windows and macOS.

Out of scope

• GIF animation — needs a renderer clock, disposal-aware compositing, and repaint hooks. Separate design pass.
• Windows C# decode callbacks — windows/Ghostty/Interop/NativeMethods.cs doesn't install any of the PNG/JPEG/GIF callbacks today. This PR inherits the gap; a separate PR can add the C# bindings.

Test plan

• zig build test passes on Windows
• zig build test passes on macOS (via SSH bundle)
• Interactive smoke with imgcat logo.gif on Wintty.exe