[2/7] Check the edit before encoding and fail on placeholders

[dazzatronus]

A host can only fall back to a cloud render if the export tells it the browser can't draw the edit faithfully. The exporter encoded whatever was on the canvas: grey placeholders for failed loads, the pending animation for prompt-only AI clips, unkeyed chroma key. Before encoding, it now checks the edit and the browser and rejects with phase unsupported and a list of issues (codes in the readme). During export, a clip still showing a placeholder, or a load still pending after 30 s, rejects with phase assets. String font weights are read the way RichTextPlayer reads them, so weight: "bold" doesn't raise a false font issue.

About 550 lines, more than half tests: one per issue code.

#	PR	Adds	Why separate
1	#224	exportBlob as the export engine	The new API
2 (this PR)	#225	Pre-flight issues; fail on placeholders	Routing depends on it
3	#226	Off-screen frames	Stops the page jumping
4	#227	Clip speed in frames and audio	Its own fix
5	#228	Size and bitrate from the edit	Its own fix
6	#229	Refuse or correct the remaining wrong exports	Found in the whole-stack review
7	#230	Release 2.25.0	Mechanical

Verify: npm run verify:ci; in a browser, a faithful edit exports in Safari and Firefox, and prompt-assets.json rejects with ai-pending and clip ids matching getEdit({ includeIds: true }).
Risk: export() refuses edits it used to encode best-effort; the changelog in [7/7] says so.