fix(core): a film moves on past a video the browser never starts loading - #7368

What

A film with a video the browser never starts loading now moves on after the usual 5 s wait, instead of freezing. The runtime skips that video and names it in a warning: skipping "…/clip.mp4": the browser never started loading it; the rest plays.

Why

Safari sometimes leaves a video at networkState LOADING and readyState HAVE_NOTHING without ever sending its request. #7347 traced this in the Safari perf runs: the server receives no request, and the page has no resource-timing entry. The runtime holds playback while a due video has no data. The skip from #7355 covers a video that stopped after its header (HAVE_METADATA and NETWORK_IDLE), but not this state, so a viewer's film held at that frame indefinitely. A one-time load() does not recover it (#7347).

Related work

• Extends fix(core): skip a video the browser can't decode instead of freezing the film #7355's skip in isUnplayable, with the same 5 s wait from the first play request and the same one-time warning.
• Follows test(player): a Safari perf run that fails says which wait timed out and why #7347, which named the same state in the Safari perf bench.

How

• media.ts: stallSinceFirstAsk returns why a video is stalled 5 s after it was first asked to play. It is either "after its header" (fix(core): skip a video the browser can't decode instead of freezing the film #7355's case) or "never started": HAVE_NOTHING and NETWORK_LOADING with no progress event since it was first asked to play.
• The progress condition keeps a slow fetch waiting. Bytes are arriving there, so the browser did start it.
• isUnplayable uses that reason in its warning. Playback, the playhead's media hold and the audio clock already skip anything isUnplayable reports, so the film moves on.
• An emptied event (a new source) clears both the wait and the bytes mark, as it already cleared the wait.
• Preview and player playback only. Renders seek frame by frame and do not use this path.

Test plan

• Unit tests added in media.test.ts: a clip at HAVE_NOTHING and NETWORK_LOADING is kept at 4999 ms, then skipped and named at 5000 ms. The same clip with a progress event is still kept at 60 s.
• Film-level test added in init.test.ts: a muted video in that state, played for 7 s of frames, moves the playhead past 1 s and logs the warning.
• Fails without the fix: with the "never started" case removed, the film test's playhead stays at 0.017 s after 7 s and the unit skip test fails. Ignoring progress fails the slow-fetch test.
• media.test.ts (190 tests) and the new film test pass 3 runs in a row. The whole runtime suite passes (2864 tests). Core typecheck, test:hyperframe-runtime-ci, oxlint, oxfmt and the comment checks are clean.

Size

One state added to the existing skip check, plus three tests.