gamut-heic: Annex-B export helpers for platform hardware decoders

[justin13888]

• Implement HEIF container parsing #238 landed the decode-ready surface (HevcDecoder, HevcConfig, NalUnitIter, grid/alpha/transform reassembly) — this is convenience on top, not a blocker.
• VideoToolbox consumes hvcC + length-prefixed samples directly; VAAPI and MediaCodec want Annex-B parameter sets / csd-0.
• Deliver: helper emitting HevcConfig NAL arrays + item payload as an Annex-B (start-code) byte stream; document per-API expectations.
• Downstream can do this locally with NalUnitIter; filed for API symmetry.
• Related: Implement HEIF container parsing #238, Stable Roadmap #168.