fix(inspect): keep a replaced video in place until its pipeline has started (#291)

WebKit (GStreamer) deadlocks the page when a video leaves the page or the
viewport before its pipeline has started: the main thread changes the
pipeline state and waits for the source's streaming thread, which waits on
the main thread in a synchronous bus handler (native stacks of the hung WPE
web process in the pinned image). Removing, hiding or merely pushing the
video below the fold all hung; none of pause(), removing src, load() or the
revoke was the cause on its own.

A video replaced while still starting now stays where it is, inert and
aria-hidden, with the new preview inserted after it, and the session's
revoke of its URL waits. Once it reports metadata or an error (or after
15 s) it is paused, loses its src, is reset with load() and removed, and
only then is its URL revoked. Every released video is reset with load(), so
no decoder is kept per replaced video.

The loadstart replacement spec now runs on WebKit too (15 fresh pages,
replaced at loadstart + 0-4 ms). TESTING.md D62 records the cause.

tools/vendor/frontend-coverage-baseline.json

@@ -296,7 +296,7 @@
   "workspace/inspect.mjs": {
     "lines": 98,
     "branches": 92,
-    "functions": 92,
+    "functions": 93,
     "statements": 98
   },
   "workspace/inspection-state.mjs": {