host/x11: default to XDamage capture; drop --untimed from viewers

X11 full-desktop capture used `ximagesrc use-damage=false`, which copies
the whole root window every frame. On servers without working MIT-SHM
(and CPU-bound everywhere else) this collapses to ~1 fps — a field test
over an xlibre host played back at roughly one frame per minute. Default
to `use-damage=true` (XDamage re-grabs only changed regions); keep
`PIXELPASS_X11_NO_DAMAGE=1` as an escape hatch for driver artifacts.

Also drop `--untimed` from both mpv invocations (viewer banner + the
interactive launcher). `--untimed` displays each frame as it decodes and
ignores audio timestamps, which drifts a shared *video* progressively
out of sync with its audio. Pacing to the audio clock keeps A/V synced
at a negligible latency cost.

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

src/host/audio.rs

@@ -280,7 +280,9 @@ impl Drop for Routing {
 /// initial `lost`. In every other mode (whole-desktop, or best-effort app
 /// filtering) the loopback keeps audio flowing from the outset, so there is no
 /// initial gap to report. Pure: no I/O, so the emit decision is unit-testable.
-pub(super) fn initial_app_audio_state(opts: &HostOpts) -> Option<crate::common::output::AppAudioState> {
+pub(super) fn initial_app_audio_state(
+    opts: &HostOpts,
+) -> Option<crate::common::output::AppAudioState> {
     (opts.app.is_some() && opts.strict_audio).then_some(crate::common::output::AppAudioState::Lost)
 }