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/windows/src/trust.rs

@@ -138,6 +138,26 @@ pub struct Settings {
     pub hdr_enabled: bool,
     /// Video decode backend: `auto` (D3D11VA, fall back to software), `hardware`, or `software`.
     pub decoder: String,
+    /// Preferred video codec: `"auto"` (host decides), `"hevc"`, `"h264"`, or `"av1"`. A soft
+    /// preference — the host honors it when it can emit it, else falls back to the best shared codec.
+    #[serde(default = "default_codec")]
+    pub codec: String,
+}
+
+fn default_codec() -> String {
+    "auto".into()
+}
+
+impl Settings {
+    /// The `codec` setting as a `quic::CODEC_*` preference bit (`0` = auto).
+    pub fn preferred_codec(&self) -> u8 {
+        match self.codec.as_str() {
+            "h264" | "avc" => punktfunk_core::quic::CODEC_H264,
+            "hevc" | "h265" => punktfunk_core::quic::CODEC_HEVC,
+            "av1" => punktfunk_core::quic::CODEC_AV1,
+            _ => 0,
+        }
+    }
 }
 
 impl Default for Settings {
@@ -154,6 +174,7 @@ impl Default for Settings {
             audio_channels: 2,
             hdr_enabled: true,
             decoder: "auto".into(),
+            codec: "auto".into(),
         }
     }
 }