feat(clients/apple): AV1 decode support — OBU plumbing, hardware-gated advertisement

The Apple client was HEVC/H.264-only: the receive path spoke Annex-B NALs
exclusively, so AV1 was never advertised and the codec picker hid it. Add
the OBU flavor of the same plumbing (AV1.swift, sibling of AnnexB.swift):
a zero-copy OBU walker, a full spec-5.5.1 sequence-header parser, an av1C
CMVideoFormatDescription with colorimetry extensions (so isHDRFormat and
the presenter stay codec-agnostic), and an ISOBMFF 'av01' sample repack
(temporal delimiter stripped, everything size-fielded, one copy per AU).

VideoCodec gains .av1 (wire 0x04); both pumps and VideoDecoder route
through dispatching formatDescription(fromKeyframe:)/sampleBuffer(au:) —
keyframe gating keys on the in-band sequence header exactly as the NAL
codecs key on in-band parameter sets, so loss recovery and mid-session
reconfigure work unchanged. AV1 sessions require a hardware decoder
(VideoToolbox has no software AV1; same fail-fast policy as 4:4:4), and
both the Hello advertisement and the Settings picker are gated on
VTIsHardwareDecodeSupported — AV1 only appears on devices that can
actually decode it (M3-class Macs, A17 Pro-class iPhones; no Apple TV).

Tests: real SVT-AV1 blobs (generation recipe in the file) cover the walk,
the parse against an independent reference, av1C bytes, delta-TU gating,
repack byte-exactness, and — on AV1 hardware — a real
VTDecompressionSession decode through VideoDecoder. Host precedence stays
HEVC > AV1 > H.264, so AV1 engages only when explicitly picked.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

clients/apple/README.md

@@ -10,7 +10,8 @@ Opus audio, cert pinning — lives in the shared Rust **`punktfunk-core`** (stat
 
 ## Features
 
-- **Hardware decode** — VideoToolbox HEVC, with a low-latency **stage-2 presenter**
+- **Hardware decode** — VideoToolbox H.264/HEVC (plus **AV1** on devices with an AV1 hardware
+  decoder — M3-class Macs, A17 Pro-class iPhones), with a low-latency **stage-2 presenter**
   (`VTDecompressionSession` → `CAMetalLayer`, presented off a `CADisplayLink`, ~11 ms p50) as the
   default and an `AVSampleBufferDisplayLayer` fallback.
 - **HDR & 4:4:4** — PQ passthrough with a correct reference-white anchor, mid-session SDR↔HDR