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/src/settings.tsx

@@ -34,6 +34,15 @@ const GAMEPAD_LABELS: Record<string, string> = {
   dualshock4: "DualShock 4",
   steamdeck: "Steam Deck",
 };
+// Mirrors the desktop client's picker (ui_settings.rs CODECS) — a soft preference the host
+// falls back from when its GPU can't encode it.
+const CODECS = ["auto", "hevc", "h264", "av1"];
+const CODEC_LABELS: Record<string, string> = {
+  auto: "Automatic",
+  hevc: "HEVC (H.265)",
+  h264: "H.264 (AVC)",
+  av1: "AV1",
+};
 const COMPOSITORS = ["auto", "kwin", "wlroots", "mutter", "gamescope"];
 const COMPOSITOR_LABELS: Record<string, string> = {
   auto: "Automatic",
@@ -108,6 +117,21 @@ export const SettingsSection: FC = () => {
         valueSuffix=" Mbit/s"
         onChange={(v) => patch({ bitrate_kbps: v * 1000 })}
       />
+      <Field
+        label="Video codec"
+        description="Preferred stream codec — the host falls back when its GPU can't encode it"
+        childrenContainerWidth="max"
+      >
+        <RowActions>
+          <div style={selectShell}>
+            <Dropdown
+              rgOptions={CODECS.map((c) => ({ data: c, label: CODEC_LABELS[c] ?? c }))}
+              selectedOption={s.codec ?? "auto"}
+              onChange={(o) => patch({ codec: o.data as string })}
+            />
+          </div>
+        </RowActions>
+      </Field>
       <Field
         label="Gamepad type"
         description="Which virtual controller the host creates for your inputs"