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/android/native/src/decode.rs

@@ -27,16 +27,23 @@ pub fn run(
 ) {
     boost_thread_priority();
     let mode = client.mode();
-    let codec = match MediaCodec::from_decoder_type("video/hevc") {
+    // The MediaCodec MIME for the codec the host resolved (`Welcome.codec`): HEVC or H.264. AMediaCodec
+    // needs no out-of-band extradata — the in-band VPS/SPS/PPS on every IDR configure it either way.
+    let mime = match client.codec {
+        punktfunk_core::quic::CODEC_H264 => "video/avc",
+        _ => "video/hevc",
+    };
+    let codec = match MediaCodec::from_decoder_type(mime) {
         Some(c) => c,
         None => {
-            log::error!("decode: no HEVC decoder on this device");
+            log::error!("decode: no {mime} decoder on this device");
             return;
         }
     };
+    log::info!("decode: codec mime = {mime}");
 
     let mut format = MediaFormat::new();
-    format.set_str("mime", "video/hevc");
+    format.set_str("mime", mime);
     format.set_i32("width", mode.width as i32);
     format.set_i32("height", mode.height as i32);
     // Generous input buffer so a large keyframe AU is never truncated.