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/PunktfunkClient/Settings/SettingsOptions.swift

@@ -38,13 +38,21 @@ enum SettingsOptions {
         HUDPlacement.allCases.map { ($0.label, $0.rawValue) }
 
     /// Video-codec preference (`DefaultsKey.codec`) — a soft preference the host falls back from.
-    /// No AV1: this client's VideoToolbox path decodes H.264/HEVC only (AnnexB.swift is NAL-only),
-    /// so it never advertises AV1 — offering it here would be a dead setting.
-    static let codecs: [(label: String, tag: String)] = [
-        ("Automatic", "auto"),
-        ("HEVC (H.265)", "hevc"),
-        ("H.264 (AVC)", "h264"),
-    ]
+    /// AV1 appears only on devices with an AV1 hardware decoder (the same
+    /// `AV1.hardwareDecodeSupported` gate SessionModel advertises by) — elsewhere it would be a
+    /// dead setting the host could never honor. Ordered by the host's resolve precedence
+    /// (HEVC > AV1 > H.264).
+    static let codecs: [(label: String, tag: String)] = {
+        var options: [(label: String, tag: String)] = [
+            ("Automatic", "auto"),
+            ("HEVC (H.265)", "hevc"),
+            ("H.264 (AVC)", "h264"),
+        ]
+        if AV1.hardwareDecodeSupported {
+            options.insert(("AV1", "av1"), at: 2)
+        }
+        return options
+    }()
 
     // MARK: - Bitrate