[1/7] Add exportBlob as the export engine

[dazzatronus]

Hosts that want to show their own progress, upload the file or cancel can't use export(): it always downloads and draws its own overlay. exportBlob({ signal, onProgress }) resolves with the MP4 Blob and draws nothing. export(filename, fps) keeps its signature and behaviour and runs on the same private engine. Aborting rejects with ExportError phase cancelled, and ExportError is exported from the package root.

#	PR	Adds	Why separate
1 (this PR)	#224	exportBlob as the export engine	The new API
2	#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; tests/export-coordinator.test.ts covers progress, abort before and mid-export, and export() still downloading.