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/Sources/PunktfunkKit/Connection/PunktfunkConnection.swift

@@ -269,7 +269,8 @@ public final class PunktfunkConnection {
     /// `2` = HEVC (default / older host), `1` = H.264, `4` = AV1. Build the decoder from THIS. The
     /// resolved value honors the client's `preferredCodec` when the host could emit it.
     public private(set) var resolvedCodec: UInt8 = 2 // PUNKTFUNK_CODEC_HEVC
-    /// The resolved codec as an `AnnexB.VideoCodec` (H.264 vs HEVC) — drives the NAL parsing.
+    /// The resolved codec as a `VideoCodec` (H.264 / HEVC / AV1) — drives the bitstream framing
+    /// (Annex-B NAL parsing vs the AV1 OBU repack).
     public var videoCodec: VideoCodec { VideoCodec(wire: resolvedCodec) }
 
     /// Connect and start a session at the requested mode (the host creates a native virtual