[3/7] Render export frames off-screen

[dazzatronus]

Exporting took over the live canvas: it reset the viewport, paused the ticker and put the editor in export mode, so the page jumped while it ran. Frames now render off-screen through pixi's extract at the output size while the app is stopped, and the view, playhead and playback come back as they were, even when a pause listener or a seek throws. ExportCommand is gone, so an export adds no undo entry and emits no edit:changed with source export. Setup, cancel and download failures keep their ExportError.

#	PR	Adds	Why separate
1	#224	exportBlob as the export engine	The new API
2	#225	Pre-flight issues; fail on placeholders	Routing depends on it
3 (this PR)	#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, export while zoomed, panned and playing: the view doesn't move, frames match an unzoomed export, and a 1080×1080 edit exports at 1080×1080 on a devicePixelRatio 2 display.
Risk: code listening for edit:changed with source export loses that event (changelog in [7/7]).