Path to v1: depend on gamut

[justin13888]

• rawshift v1 replaces its codec/metadata/TIFF internals with the gamut crate family, consumed as a git dependency pinned to a master commit hash (currently 295c8924).
• Policy: upstream-first — any rawshift change requiring gamut changes files a gamut issue first, waits for it to land, bumps the pin, then changes rawshift (see AGENTS.md). Hardening doubts go upstream as chore issues.
• Permanent exceptions (never migrate): GIF (gif), SVG (resvg), PPM (zune-ppm).
• v1 ships image only; video is parked. HEIC/AVIF pixel decode is hardware-only (VideoToolbox/VAAPI/MediaCodec) per the permanent matrix in docs/SUPPORT.md; full design in docs/V1_DESIGN.md.
• Upstream v1 gates: Implement full spec-compliant JPEG encoder/decoder gamut#28 (JPEG), gamut-png: implement spec-compliant PNG decoder gamut#249 (PNG decode), gamut-avif: container read surface + AV1 codestream handoff for external decoders gamut#250 (AVIF handoff), gamut-ifd: RAW-grade parsing for camera TIFF files gamut#252 + #262/#263 (gamut-ifd), gamut-color: add BitDepth::Sixteen gamut#260 (BitDepth::Sixteen), gamut-dng: public bridge surface for external RAW pipelines gamut#253 (DNG bridge slices).

Phase 0 — policy + pin

• Adopt upstream-first gamut policy, permanent support matrix, and MSRV policy #16
• Add gamut as pinned git dependency #17
• Park rawshift-video for v1 #36

Phase 1 — core primitives

• rawshift-core: replace generic primitives with gamut re-exports, keep sensor types #18

Phase 2 — metadata + TIFF spine

• Migrate metadata stack to gamut-metadata #19
• Migrate TIFF structure parser from binrw to gamut-ifd #20
• Migrate DNG decode/encode to gamut-dng #21
• Migrate TIFF codec to gamut-tiff and expose TIFF encode #22

Phase 3 — unblocked codecs + hardware decode

• Migrate PNG encode to gamut-png #23
• Migrate WebP to gamut-webp #24
• Migrate JXL to gamut-jxl #25
• Migrate AVIF encode to gamut-avif; delete libaom and ravif #26
• Create rawshift-hwdec (HEVC+AV1); HEIC on gamut-heic; drop libheif-rs #27
• Hardware decode: VideoToolbox backend (macOS/iOS, HEVC + AV1) #28
• Hardware decode: VAAPI backend (Linux, HEVC + AV1, dlopen) #29
• Hardware decode: MediaCodec backend (Android, HEVC + AV1) #30

Phase 4 — upstream-gated codecs (one pin bump per landing)

• Migrate JPEG to gamut #31
• Migrate PNG decode to gamut-png #32
• AVIF decode via gamut-avif container + hardware AV1 #33

Phase 5 — v1 finalization

• Feature tree + compile-boundary CI rework post-migration #34
• Test fixtures and benchmark updates for the gamut migration #35
• Finalize v1 high-level image API and release #37