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>

packaging/README.md

@@ -1,8 +1,9 @@
 # Packaging punktfunk for Fedora / Bazzite
 
-The punktfunk host is Linux-only and links system FFmpeg (NVENC), PipeWire, Opus and
-the NVIDIA driver. This directory packages it for the **Fedora Atomic / Bazzite** world
-(rpm-ostree + bootc), where most of those deps are already present.
+The punktfunk host links system FFmpeg (NVENC on NVIDIA, VAAPI on AMD/Intel, with a GPU-less
+software-H.264 fallback), PipeWire and Opus. This page covers packaging it for the
+**Fedora Atomic / Bazzite** world (rpm-ostree + bootc), where most of those deps are already
+present; the NVIDIA-specific notes below apply to the NVENC path.
 
 > 👉 **Ubuntu/Debian hosts** install via `apt` from Gitea's package registry — see
 > [`debian/README.md`](debian/README.md) (`apt update && apt upgrade` for new builds).
@@ -20,6 +21,10 @@ packaging/
   copr/                   # COPR build-from-SCM settings
 ```
 
+The other packaging targets have their own READMEs: [`debian/`](debian/README.md) (apt),
+[`arch/`](arch/README.md) (PKGBUILD + sysext), [`flatpak/`](flatpak/README.md) (the client),
+[`windows/`](windows/README.md) (host installer + drivers), plus `kde/` and `linux/` helpers.
+
 ## What's needed beyond base Fedora
 
 | Dependency | Where it comes from |
@@ -121,7 +126,8 @@ An RPM (or the bootc layer) installs into the host system where those just work.
 ## Building the SRPM/RPM locally (Fedora only)
 
 ```sh
-git archive --format=tar.gz --prefix=punktfunk-0.0.1/ -o ~/rpmbuild/SOURCES/punktfunk-0.0.1.tar.gz HEAD
+git archive --format=tar.gz --prefix=punktfunk-0.3.0/ -o ~/rpmbuild/SOURCES/punktfunk-0.3.0.tar.gz HEAD
 rpmbuild -ba packaging/rpm/punktfunk.spec     # needs the BuildRequires from the spec
+# (0.3.0 = the spec's default %{pf_version}; the prefix and tarball name must match it)
 ```
 (Not buildable on Debian/Ubuntu — use a Fedora toolbox/container or COPR.)