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/PunktfunkKit/StreamPump.swift

@@ -95,7 +95,7 @@ final class StreamPump {
 
                     guard let au = try connection.nextAU(timeoutMs: 100) else { continue }
                     onFrame?(au)
-                    let idrFormat = AnnexB.formatDescription(fromIDR: au.data)
+                    let idrFormat = AnnexB.formatDescription(fromIDR: au.data, codec: connection.videoCodec)
                     if let f = idrFormat {
                         format = f          // refreshed on every IDR (mode changes included)
                         if awaitingIDR {
@@ -119,7 +119,7 @@ final class StreamPump {
                     }
                     wasFailed = failed
                     guard let f = format,
-                          let sample = AnnexB.sampleBuffer(au: au, format: f),
+                          let sample = AnnexB.sampleBuffer(au: au, format: f, codec: connection.videoCodec),
                           token.isLive // don't enqueue a stale frame after a restart
                     else { continue }
                     layer.enqueue(sample)