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>

docs-site/content/docs/roadmap.md

@@ -52,8 +52,8 @@ see [Status & Progress](/docs/status).
 
 - **Windows client on-glass validation.** The hardware (D3D11VA) decode, HDR present, and GUI are
   built and ship as a signed MSIX — they just need verification on real GPU hardware.
-- **Apple stage-2 presenter as the default.** The lower-latency `VTDecompressionSession` →
-  `CAMetalLayer` path is live behind an opt-in flag and graduating to the default.
+- **Apple presenter polish.** The lower-latency `VTDecompressionSession` → `CAMetalLayer` stage-2
+  path is now the default; HDR brightness and 4:4:4 still need on-glass validation.
 - **Web console parity.** Surfacing the speed test and bitrate picker the apps already have.
 - **Windows host hardening.** Broader real-world testing — especially on-glass validation of the
   AMD (AMF) and Intel (QSV) encode paths, which are CI-green but newer than NVENC.