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/requirements.md

@@ -5,7 +5,8 @@ description: What you need to run a punktfunk host — GPU, driver, desktop, and
 
 ## Supported setups
 
-A punktfunk host runs primarily on a Linux machine with an NVIDIA GPU (a native
+A punktfunk host runs primarily on a Linux machine with a dedicated GPU — NVIDIA (NVENC) is the
+most-exercised path, and AMD/Intel GPUs work via VAAPI (a native
 [Windows host](/docs/windows-host) is also available — see below). These are the Linux desktop
 environments it supports today, each with its own guide:
 
@@ -32,6 +33,10 @@ listed, the host still needs one of these compositor backends to create a virtua
   not just `nvidia-utils` — without it the compositor can't initialise the GPU and capture fails. Each
   setup guide installs the right package (e.g. `libnvidia-gl-<version>` on Ubuntu).
 - **`nvidia-drm modeset=1`** must be enabled (Wayland on NVIDIA needs it). The setup guides cover this.
+- **AMD / Intel GPUs** encode via **VAAPI** instead (install `mesa-va-drivers` or
+  `intel-media-driver`; validated live on AMD RDNA3). The NVIDIA-specific notes above don't apply
+  there. A GPU-less software H.264 encoder also exists (`PUNKTFUNK_ENCODER=software`), meant as a
+  fallback rather than a daily driver.
 
 > Consumer GeForce cards historically cap the number of **concurrent** NVENC sessions (a few at once);
 > workstation cards don't. This only matters if you stream to many devices simultaneously.