feat(protocol,host): negotiate video codec + add a GPU-less software (openh264) encode path

Phase 1 of codec negotiation, and the Linux software H.264 encode path it unblocks.

**Codec negotiation (core `quic`):**
- `Hello.video_codecs` (bitfield: CODEC_H264/HEVC/AV1) — the client advertises what it can
  decode; appended as a trailing byte (older client → 0 = HEVC-only, back-compat).
- `Welcome.codec` — the single codec the host resolved and will emit; trailing byte (older
  host → HEVC).
- `resolve_codec(client, host_capable)` picks the shared codec (precedence HEVC > AV1 > H.264)
  or `None` → the host refuses honestly rather than sending an undecodable stream.
- Roundtrip + back-compat tests; cbindgen exports the CODEC_* constants.

**Software encoder (host):**
- The openh264 `OpenH264Encoder` (was Windows-only) is now built on Linux too — it's
  platform-agnostic (consumes CPU RGB `CapturedFrame`s, statically-bundled openh264). `openh264`
  moved to the shared linux+windows Cargo target.
- `PUNKTFUNK_ENCODER=software` selects it: `open_video` gains a `software` branch (H.264 only),
  and `session_plan::resolve_encoder` / `capture::gpu_encode` resolve `EncoderBackend::Software`
  → `output_format().gpu = false`, so the portal capturer delivers CPU RGB. Explicit-only (auto
  never picks it — a box with a dead driver still has /dev/nvidiactl and would mis-resolve NVENC).

**Host codec resolution (`punktfunk1`):**
- The native path no longer hardcodes HEVC: it resolves the codec from the client's advertised
  set ∩ the host's capability (`Codec::host_wire_caps`: software→H.264, else HEVC), threads it
  through `SessionPlan.codec`, and opens the encoder + validates reconfigures at that codec. A
  software host + HEVC-only client is refused with a clear error.
- 4:4:4 is gated on HEVC (it's HEVC-only).

**Probe:** advertises H264|HEVC|AV1 and logs the resolved codec.

Validated on the GPU-less dev box: negotiation is live end-to-end (probe advertises 0x07 → host
resolves H.264 → Welcome reports it → plan = Software/H264), and the openh264 unit test (CPU RGB →
AnnexB IDR) now runs on Linux. Full capture→encode still needs a GPU on this box — every
compositor screencast path (KWin GL, gamescope VK_EXT_physical_device_drm, wlroots EGL) requires
one; software render (llvmpipe/pixman) can't be captured — so this box exercises negotiation +
encoder, not live capture. The software path unblocks GPU-less-*encode* boxes that still have a
display GPU. Phase 2 (clients advertising real codecs + decoding per Welcome.codec) is a follow-up.

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

clients/probe/src/main.rs

@@ -231,6 +231,16 @@ fn now_ns() -> u64 {
         .unwrap_or(0)
 }
 
+/// Human name for the negotiated `Welcome::codec` (also the natural `--out` file extension). The
+/// bitstream is dumped verbatim, so an H.264 software-host session should be saved as `.h264`.
+fn codec_ext(codec: u8) -> &'static str {
+    match codec {
+        punktfunk_core::quic::CODEC_H264 => "h264",
+        punktfunk_core::quic::CODEC_AV1 => "av1",
+        _ => "h265",
+    }
+}
+
 fn main() {
     tracing_subscriber::fmt()
         .with_env_filter(
@@ -411,6 +421,13 @@ async fn session(args: Args) -> Result<()> {
             // `--audio-channels` (default stereo); the probe multistream-decodes + validates the
             // host's frames to exercise the surround encode path headlessly.
             audio_channels: args.audio_channels,
+            // The probe just dumps the bitstream (no decode), so it advertises every codec — HEVC
+            // (the host default) AND H.264 (so it can drive a GPU-less software host,
+            // `PUNKTFUNK_ENCODER=software`) AND AV1. The host picks one and reports it in
+            // `Welcome::codec`; the dump extension follows that.
+            video_codecs: punktfunk_core::quic::CODEC_H264
+                | punktfunk_core::quic::CODEC_HEVC
+                | punktfunk_core::quic::CODEC_AV1,
         }
         .encode(),
     )
@@ -429,6 +446,7 @@ async fn session(args: Args) -> Result<()> {
         hdr = welcome.color.is_hdr(),
         chroma_444 = welcome.chroma_format == punktfunk_core::quic::CHROMA_IDC_444,
         chroma_format_idc = welcome.chroma_format,
+        codec = codec_ext(welcome.codec),
         "session offer"
     );