`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>