Create rawshift-hwdec (HEVC+AV1); HEIC on gamut-heic; drop libheif-rs

[justin13888]

• New leaf crate rawshift-hwdec: all platform FFI; safe public API — HwCodec{Hevc,Av1}, HwBackend, StillDecodeRequest (hvcC|av1C config + NAL|OBU payload), DecodedFrame (Nv12/P010/I420/I010 planes), HwStillDecoder, decoder(codec), backend(), available_codecs(); #![deny(unsafe_op_in_unsafe_fn)] per PRINCIPLES.md.
• Verified feature flags: videotoolbox/vaapi/mediacodec hard-compile_error! on foreign targets; portable hw meta-flag selects the native backend, build-script warning + stub on no-API targets (see docs/SUPPORT.md).
• Rewrite formats/heic.rs on gamut-heic (HeifImage::decode_item_planar, NalUnitIter, hevc_config(), EXIF/XMP items — all landed upstream in gamut#238; no upstream blocker; gamut#18 is a patent-expiry placeholder, NOT a dependency); tile stitch + irot/imir/clap in safe Rust; YCbCr→RGB via gamut-color.
• Add heic_hw_decode_available(); RawError::HwDecoderUnavailable{codec,reason}; container/metadata/aux enumeration work without any backend.
• Drop libheif-rs + heic-vendored; rewire heic_inspect example + heic_aux test.
• Per-platform backends are sub-issues (VideoToolbox / VAAPI / MediaCodec).
• Depends on: rawshift-core: replace generic primitives with gamut re-exports, keep sensor types #18, Migrate metadata stack to gamut-metadata #19.