feat: pure Rust UltraHDR implementation (RIIR)

[Enter-tainer]

Summary

Rewrite the UltraHDR gain-map JPEG library in pure Rust, eliminating all C/C++ FFI dependencies (cmake, nasm, pkg-config, bindgen).

What's new

• Encoder: HDR+SDR → UltraHDR JPEG encoding with gain map generation, ICC profiles, and MPF assembly. Supports HDR-only auto tone-map (API-0 scenario).
• Decoder: UltraHDR JPEG → packed pixel buffer (RGBA8888, RGBA1010102, RGBAF16). Gain map metadata probing without full decode.
• Color: sRGB/PQ/HLG transfer functions, BT.709 ↔ Display P3 ↔ BT.2100 gamut conversion, ICC profile parsing & generation.
• Gain Map: ISO 21496-1 binary metadata encode/decode, XMP metadata read/write, bilinear interpolation sampling, Reinhard tone mapping.
• JPEG: Segment parser, codec wrappers (jpeg-decoder + jpeg-encoder).
• MPF: Multi-Picture Format segment generation.
• CLI: ultrahdr-bake migrated to pure Rust API (bake, motion photo, auto-detect preserved).

Key properties

• Zero unsafe code in the entire crate
• 3 runtime dependencies only: jpeg-decoder, jpeg-encoder, roxmltree
• 73 tests (71 unit + 2 integration roundtrip), all passing
• Zero clippy warnings (-D warnings)
• Zero cargo doc warnings
• Builder pattern API for both Encoder and Decoder

Modules

ultrahdr/src/
├── encoder.rs       — Builder API, gain map generation, JPEG assembly
├── decoder.rs       — Gain map extraction, HDR reconstruction
├── gainmap/
│   ├── math.rs      — Gain computation, bilinear sampling, tone mapping
│   └── metadata.rs  — ISO 21496-1 + XMP metadata
├── color/
│   ├── mod.rs       — Color type with arithmetic ops
│   ├── gamut.rs     — Gamut conversion matrices
│   ├── transfer.rs  — sRGB/PQ/HLG OETF/inv-OETF
│   └── icc.rs       — ICC profile parse & generate
├── jpeg/
│   ├── mod.rs       — JPEG segment parser
│   ├── decode.rs    — jpeg-decoder wrapper
│   └── encode.rs    — jpeg-encoder wrapper
├── mpf.rs           — Multi-Picture Format
├── types.rs         — Core types (PixelFormat, ColorGamut, etc.)
└── error.rs         — Error types