feat(clients): codec preference on Windows/Apple/Android clients (Phase 2b)

Rounds out codec negotiation across the last three clients — each advertises what it can decode,
builds its decoder from the resolved Welcome.codec, and exposes a "Video codec" preference picker.

**Windows** (Rust, mirrors Linux): `decodable_codecs()` + `ffmpeg_codec_id()`; the D3D11VA and
software FFmpeg decoders (and the mid-session D3D11VA→software demotion) open the negotiated codec
instead of hardcoding HEVC; settings gain a `codec` field + reactor ComboBox; `--codec` CLI flag.

**Apple** (Swift/C-ABI): AnnexB is now codec-aware — a `VideoCodec` enum drives H.264 vs HEVC NAL
parsing / parameter-set extraction (`CMVideoFormatDescriptionCreateFromH264ParameterSets` for H.264,
no VPS) and AVCC repacking; `PunktfunkConnection` advertises H264|HEVC via `punktfunk_connect_ex7`,
reads `resolvedCodec` (`punktfunk_connection_codec`), and threads `videoCodec` into the stage-1/2
pipelines + `VideoDecoder`; SettingsView "Video codec" Picker (auto/HEVC/H.264). AV1 is left out
(hosts don't emit it on the native path, and it's not an AnnexB codec). Test call sites updated.

**Android** (Kotlin + Rust JNI): the JNI `nativeConnect` gains `preferredCodec`; the native decode
loop picks the AMediaCodec MIME (`video/hevc`|`video/avc`) from `connector.codec` and advertises
H264|HEVC; Settings `codec` field + Compose dropdown.

Core/host/probe/Linux clippy + tests green (unchanged from 2a). Windows/Apple/Android compile on
their platform CI (this Linux box can't build them — Windows toolchain / Xcode / the Android NDK's
opus-cmake toolchain). All follow the Linux client's validated pattern.

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

clients/apple/Sources/PunktfunkClient/ContentView.swift

@@ -28,10 +28,20 @@ struct ContentView: View {
     @AppStorage(DefaultsKey.gamepadType) private var gamepadType = 0
     @AppStorage(DefaultsKey.bitrateKbps) private var bitrateKbps = 0
     @AppStorage(DefaultsKey.audioChannels) private var audioChannels = 2
+    @AppStorage(DefaultsKey.codec) private var codec = "auto"
     @AppStorage(DefaultsKey.hdrEnabled) private var hdrEnabled = true
     @AppStorage(DefaultsKey.fullscreenWhileStreaming) private var fullscreenWhileStreaming = true
     @AppStorage(DefaultsKey.hudEnabled) private var hudEnabled = true
     @AppStorage(DefaultsKey.hudPlacement) private var hudPlacement = HUDPlacement.topTrailing.rawValue
+    /// The `codec` setting as a `PUNKTFUNK_CODEC_*` soft-preference byte (`0` = auto).
+    private var preferredCodecByte: UInt8 {
+        switch codec {
+        case "h264": return PunktfunkConnection.codecH264
+        case "hevc": return PunktfunkConnection.codecHEVC
+        case "av1": return PunktfunkConnection.codecAV1
+        default: return 0
+        }
+    }
     @State private var showAddHost = false
     @State private var pairingTarget: StoredHost?
     /// A fresh `pair=required`/unknown host the user tapped: drives the choice between no-PIN
@@ -378,6 +388,7 @@ struct ContentView: View {
             bitrateKbps: UInt32(clamping: bitrateKbps),
             audioChannels: UInt8(clamping: audioChannels),
             hdrEnabled: hdrEnabled,
+            preferredCodec: preferredCodecByte,
             launchID: launchID,
             allowTofu: allowTofu,
             requestAccess: requestAccess)
@@ -521,6 +532,7 @@ struct ContentView: View {
             bitrateKbps: bitrate,
             audioChannels: UInt8(clamping: audioChannels),
             hdrEnabled: hdrEnabled,
+            preferredCodec: preferredCodecByte,
             autoTrust: true)
     }
 }

clients/apple/Sources/PunktfunkClient/SessionModel.swift

@@ -108,6 +108,7 @@ final class SessionModel: ObservableObject {
                  bitrateKbps: UInt32 = 0,
                  audioChannels: UInt8 = 2,
                  hdrEnabled: Bool = true,
+                 preferredCodec: UInt8 = 0,
                  launchID: String? = nil,
                  allowTofu: Bool = false,
                  autoTrust: Bool = false,
@@ -155,12 +156,17 @@ final class SessionModel: ObservableObject {
             if want444, canDecode444 {
                 videoCaps |= PunktfunkConnection.videoCap444
             }
+            // This client's VideoToolbox path decodes H.264 and HEVC (AV1 isn't wired — hosts don't
+            // emit it on the native path yet). The host resolves the emitted codec from these + the
+            // soft `preferredCodec`; `resolvedCodec` reflects what it chose.
+            let videoCodecs = PunktfunkConnection.codecH264 | PunktfunkConnection.codecHEVC
             let result = Result { try PunktfunkConnection(
                 host: host.address, port: host.port,
                 width: width, height: height, refreshHz: hz,
                 pinSHA256: pin, identity: identity, compositor: compositor,
                 gamepad: gamepad, bitrateKbps: bitrateKbps, videoCaps: videoCaps,
-                audioChannels: audioChannels, launchID: launchID,
+                audioChannels: audioChannels,
+                videoCodecs: videoCodecs, preferredCodec: preferredCodec, launchID: launchID,
                 // Delegated approval: the host holds this connect open until the operator approves
                 // it (~180 s) — outwait that window so a slow approval still lands here. Normal
                 // connects keep the snappy default.

clients/apple/Sources/PunktfunkClient/SettingsView.swift

@@ -30,6 +30,7 @@ struct SettingsView: View {
     @AppStorage(DefaultsKey.fullscreenWhileStreaming) private var fullscreenWhileStreaming = true
     @AppStorage(DefaultsKey.micEnabled) private var micEnabled = true
     @AppStorage(DefaultsKey.audioChannels) private var audioChannels = 2
+    @AppStorage(DefaultsKey.codec) private var codec = "auto"
     @AppStorage(DefaultsKey.hudEnabled) private var hudEnabled = true
     @AppStorage(DefaultsKey.hudPlacement) private var hudPlacement = HUDPlacement.topTrailing.rawValue
     @ObservedObject private var gamepads = GamepadManager.shared
@@ -671,16 +672,22 @@ struct SettingsView: View {
 
     @ViewBuilder private var hdrSection: some View {
         Section {
+            Picker("Video codec", selection: $codec) {
+                Text("Automatic").tag("auto")
+                Text("HEVC (H.265)").tag("hevc")
+                Text("H.264 (AVC)").tag("h264")
+            }
             Toggle("10-bit HDR", isOn: $hdrEnabled)
             Toggle("Full chroma (4:4:4)", isOn: $enable444)
         } header: {
             Text("Video quality")
         } footer: {
-            Text("HDR requests a 10-bit BT.2020 PQ (HDR10) stream — it only engages when the host is "
-                + "sending HDR content AND this display supports HDR. 4:4:4 requests full chroma "
-                + "(sharper text/UI, more bandwidth) — it only engages when this device can "
-                + "hardware-decode it AND the host opted in. Otherwise the stream stays 8-bit "
-                + "4:2:0 SDR. Applies from the next session.")
+            Text("Codec is a preference — the host falls back if it can't encode the one you pick "
+                + "(and 10-bit/4:4:4 are HEVC-only). HDR requests a 10-bit BT.2020 PQ (HDR10) stream — "
+                + "it only engages when the host is sending HDR content AND this display supports HDR. "
+                + "4:4:4 requests full chroma (sharper text/UI, more bandwidth) — it only engages when "
+                + "this device can hardware-decode it AND the host opted in. Otherwise the stream stays "
+                + "8-bit 4:2:0 SDR. Applies from the next session.")
                 .font(.geist(12, relativeTo: .caption))
                 .foregroundStyle(.secondary)
         }