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/viewer/mod.rs

@@ -102,7 +102,7 @@ fn print_viewer_banner(url: &str) {
     eprintln!("│  Connected to host. Open the stream in your player:");
     eprintln!("│");
     eprintln!(
-        "│    mpv --profile=low-latency --untimed --hwdec=auto --audio-buffer=0.2 --demuxer-max-bytes=2M --demuxer-readahead-secs=0.5 {url}"
+        "│    mpv --profile=low-latency --hwdec=auto --audio-buffer=0.2 --demuxer-max-bytes=2M --demuxer-readahead-secs=0.5 {url}"
     );
     eprintln!("│    vlc --network-caching=200 --live-caching=200 {url}");
     eprintln!("│");