Hardware decode: VideoToolbox backend (macOS/iOS, HEVC + AV1)

[justin13888]

• HwStillDecoder impl over VTDecompressionSession (VideoToolbox/CoreMedia/CoreVideo system frameworks), cfg-gated target_os = "macos"/"ios".
• HEVC: hvcC via CMVideoFormatDescription + length-prefixed samples directly (no Annex-B conversion).
• AV1: av1C config; runtime hardware probe (M3+/A17 Pro+); absent hardware → available_codecs() omits Av1.
• Map CVPixelBuffer output (NV12/P010, 8/10-bit 4:2:0) to DecodedFrame.
• Document the objc/core-foundation binding choice; macOS CI smoke test with fixture.
• Depends on: Create rawshift-hwdec (HEVC+AV1); HEIC on gamut-heic; drop libheif-rs #27.