feat(protocol,clients): codec preference negotiation + Linux client decodes per Welcome (Phase 2a)

Adds a client-selectable **preferred codec** and wires the core + ABI + probe + Linux client to
negotiate and decode it. (Windows/Apple/Android follow in 2b.)

**Core:**
- `Hello.preferred_codec` (a single CODEC_* bit, 0 = auto) — a soft hint appended after
  `video_codecs`. `resolve_codec(client, host, preferred)` now honors the preference when the host
  can also emit it, else falls back to precedence (HEVC > AV1 > H.264). Roundtrip + preference tests.
- `NativeClient::connect` takes `video_codecs` + `preferred_codec`; `NativeClient.codec` exposes the
  resolved `Welcome.codec`.
- ABI: `punktfunk_connect_ex7` (adds the two codec params; `ex6` delegates to it advertising
  HEVC-only) + `punktfunk_connection_codec` getter + `PUNKTFUNK_CODEC_{H264,HEVC,AV1}` constants
  (drift-guarded against the wire values). Header regenerated.

**Host:** passes `hello.preferred_codec` into `resolve_codec`.

**probe:** `--codec h264|hevc|av1|auto` sets the preference (still advertises it can decode all
three); the dump extension already follows the resolved codec.

**Linux client:** advertises the codecs FFmpeg can actually decode (`decodable_codecs()`), threads
the user's `codec` setting as the preference, and builds the decoder — both the software and VAAPI
paths, plus the mid-session VAAPI→software demotion — from the negotiated `Welcome.codec` instead of
hardcoding HEVC. New "Video codec" dropdown in Preferences (Automatic/HEVC/H.264/AV1).

Live-validated on the dev box: probe `--codec hevc` against a software (H.264-only) host resolves to
H.264 (graceful soft-preference fallback), no failure. clippy + core (57) + host (133) tests green.

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

clients/linux/src/app.rs

@@ -529,9 +529,11 @@ fn speed_test(app: Rc<App>, req: ConnectRequest) {
                 },
                 CompositorPref::Auto,
                 GamepadPref::Auto,
-                0,    // bitrate_kbps (host default)
-                0,    // video_caps: the Linux client has no 10-bit/HDR present path yet
-                2,    // audio_channels: speed-test probe, stereo
+                0,                                // bitrate_kbps (host default)
+                0, // video_caps: the Linux client has no 10-bit/HDR present path yet
+                2, // audio_channels: speed-test probe, stereo
+                crate::video::decodable_codecs(), // codecs (unused by the probe, but honest)
+                0, // preferred_codec: no preference for a speed-test probe
                 None, // launch: speed-test probe connect, no game
                 pin,
                 Some(identity),
@@ -689,6 +691,7 @@ fn start_session_with(app: Rc<App>, req: ConnectRequest, pin: Option<[u8; 32]>,
         bitrate_kbps: s.bitrate_kbps,
         mic_enabled: s.mic_enabled,
         audio_channels: s.audio_channels,
+        preferred_codec: s.preferred_codec(),
         pin,
         identity: app.identity.clone(),
         connect_timeout: opts.connect_timeout,