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>

crates/punktfunk-host/Cargo.toml

@@ -71,6 +71,10 @@ tempfile = "3"
 # MSVC too (needs CMake + NASM, both on the box). Both platforms that have an audio-capture backend.
 [target.'cfg(any(target_os = "linux", target_os = "windows"))'.dependencies]
 opus = "0.3"
+# Software H.264 encoder — the GPU-less encode path on both Linux and Windows (and a fallback when no
+# hardware encoder is available). The default `source` feature statically compiles OpenH264 (BSD-2) —
+# no system lib, builds on MSVC; nasm on PATH adds the SIMD fast path.
+openh264 = "0.9"
 
 [target.'cfg(target_os = "linux")'.dependencies]
 # `screencast` gates the ScreenCast portal module; `remote_desktop` adds the RemoteDesktop
@@ -198,9 +202,6 @@ winreg = "0.56"
 # Parse each Xbox/Game-Pass game's MicrosoftGame.config (GDK manifest XML) for the Xbox store
 # provider — a small read-only DOM is all we need (Identity/Executable/ShellVisuals/StoreId).
 roxmltree = "0.21"
-# Software H.264 encoder (GPU-less path + NVENC fallback). The default `source` feature statically
-# compiles OpenH264 (BSD-2) — no system lib, builds on MSVC; nasm on PATH adds the SIMD fast path.
-openh264 = "0.9"
 # WASAPI loopback audio capture (default render endpoint -> 48 kHz stereo f32 for the Opus path).
 wasapi = "0.23"
 # Virtual Xbox 360 gamepad: the in-tree XUSB companion UMDF driver (packaging/windows/xusb-driver),