fix(host-control-mode): re-query host pos when captured target is unusable

Audit follow-up on the snap-back paths. The immediate involuntary snap used the
captured target directly; if it was null/invalid (host position not known yet,
e.g. before the first host heartbeat) hcmSnapBackToHost no-ops and the guest is
left silently paused with no dialog and no retry. The deferred and "Stay in sync"
paths already re-query with retry — make the immediate path fall back to the same
when the captured target isn't usable.

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

extension/content.js

@@ -261,7 +261,12 @@
             // Buffering/ads/throttle — silently re-sync, no dialog spam.
             const video = findVideo();
             if (video && video.readyState >= 3 && !video.seeking) {
-                hcmSnapBackToHost(target);        // ready now → snap immediately
+                // Ready now → snap immediately. Use the captured target if it's
+                // usable, otherwise re-query+retry (host state may not be known yet)
+                // so we never leave the guest silently stuck (consistent with the
+                // deferred and "Stay in sync" paths).
+                if (target && Number.isFinite(target.targetTime)) hcmSnapBackToHost(target);
+                else hcmRequestHostSyncWithRetry();
             } else {
                 hcmDeferredSnapBack();             // buffering → wait for ready, then snap once (#3)
             }