fix(host/rtsp): scope the HDR mut allow so Windows clippy stays green

`let mut hdr` tripped -D unused_mut on Windows: the only reassignment is the
GNOME colour-mode probe below it, which is #[cfg(target_os = "linux")]. Dropping
`mut` would break the Linux build, so allow unused_mut on non-Linux only — the
lint still fires on Linux if that probe is ever removed.

Second Windows-only breakage in this release that the Linux CI cannot see (see
also the ffmpeg_win swscale match): `cargo clippy --workspace` on the Linux
runner never compiles cfg(windows) code, and the Windows job builds only
punktfunk-host + punktfunk-tray.

Verified: cargo clippy --release -p punktfunk-host --features nvenc,amf-qsv,qsv
-- -D warnings on 192.168.1.173.

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

crates/punktfunk-host/src/gamestream/rtsp.rs

@@ -406,6 +406,9 @@ fn stream_config(map: &HashMap<String, String>) -> Option<StreamConfig> {
     // degrades to 8-bit SDR (and a Windows desktop that is ALREADY HDR still streams PQ
     // regardless, since the IDD-push capturer follows the display).
     let hdr_requested = parse_u("x-nv-video[0].dynamicRangeMode").unwrap_or(0) != 0;
+    // `mut` is load-bearing on Linux only — the GNOME colour-mode probe below clears it. Scope the
+    // allow to non-Linux so `unused_mut` still fires here if that probe ever goes away.
+    #[cfg_attr(not(target_os = "linux"), allow(unused_mut))]
     let mut hdr = hdr_requested && crate::gamestream::host_hdr_capable();
     if hdr_requested && !hdr {
         tracing::warn!(