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/interactive.rs

@@ -279,8 +279,9 @@ impl Player {
             Player::Mpv => crate::common::process::spawn_detached(
                 "mpv",
                 &[
+                    // No `--untimed`: it ignores audio timestamps and drifts a
+                    // shared video out of sync. Pacing to audio keeps A/V synced.
                     "--profile=low-latency",
-                    "--untimed",
                     "--hwdec=auto",
                     "--audio-buffer=0.2",
                     "--demuxer-max-bytes=2M",