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>

scripts/steamdeck/README.md

@@ -39,9 +39,13 @@ git clone https://git.unom.io/unom/punktfunk ~/punktfunk
 bash ~/punktfunk/scripts/steamdeck/install.sh            # PIN pairing required (secure default)
 bash ~/punktfunk/scripts/steamdeck/install.sh --open     # trusted LAN: accept unpaired clients
 bash ~/punktfunk/scripts/steamdeck/install.sh --no-web   # host only, no web console
+bash ~/punktfunk/scripts/steamdeck/install.sh --no-gamestream  # native punktfunk/1 only, no Moonlight surface
 bash ~/punktfunk/scripts/steamdeck/update.sh             # after pulling new source
 ```
 
+Note: unlike a bare `serve` (native-only by default), the Deck install enables `--gamestream` by
+default so stock Moonlight clients work out of the box; `--no-gamestream` turns that surface off.
+
 Env overrides: `PUNKTFUNK_SRC` (source dir, default `~/punktfunk`), `PUNKTFUNK_BOX` (container name,
 default `pf2`), `PUNKTFUNK_MGMT_PORT` (47990), `PUNKTFUNK_WEB_PORT` (3000).