feat(codec): make client codec selection real — GPU-aware native advertisement

The clients' codec pickers sent a preference the host threw away:
host_wire_caps() hardcoded HEVC for every GPU backend, so the native
path never emitted H.264/AV1 regardless of what the user chose.

Host: advertise what the backend actually encodes, mirroring the
GameStream serverinfo logic — software openh264 emits H.264; probed
backends (Linux VAAPI, Windows AMF/QSV) advertise their per-GPU
CodecSupport via the new wire_mask(); NVENC keeps the
Moonlight-validated H.264|HEVC|AV1 static superset; an empty probe
falls back to the superset so auto clients still resolve HEVC. Gate
10-bit to HEVC (like the 4:4:4 gate) now that a client can steer the
codec — Main10 is the only 10-bit encode path. Fix the web-console
stats label hardcoded to "hevc" (new Codec::label(), shared with the
GameStream register_session mapping).

Android: replace the hardcoded H264|HEVC Hello advertisement with a
videoCodecs param fed by VideoDecoders.decodableCodecBits() (AV1 bit
only when a real hardware video/av01 decoder exists — the decode loop
was already mime-driven); offer AV1 in the codec picker on capable
devices.

Decky: add the missing "Video codec" dropdown (auto/hevc/h264/av1) to
the plugin settings, round-tripping the same codec key the flatpak
client reads.

Apple: unchanged by design (AnnexB.swift is NAL-only, AV1 is never
advertised); refresh the stale "hosts don't emit AV1 on the native
path yet" comments here and host-side.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

clients/decky/main.py

@@ -669,7 +669,7 @@ class Plugin:
             # The client's own defaults (native display, host-default bitrate, auto pad).
             return {
                 "width": 0, "height": 0, "refresh_hz": 0, "bitrate_kbps": 0,
-                "gamepad": "auto", "compositor": "auto",
+                "codec": "auto", "gamepad": "auto", "compositor": "auto",
                 "inhibit_shortcuts": True, "mic_enabled": False,
             }