D677 — The growth replay exports as a browser-recorded video

Decision. The Atlas records the growth replay (D367) in the browser with MediaRecorder and downloads the clip: MP4 (H.264) where the browser records it, WebM where it does not. Each frame is composed on a 2D canvas, the scene's WebGL canvas plus an overlay drawn in the frame itself (KB name, replayed date, concept count, an end card), at 16:9, 9:16 or 1:1, chosen at export time. The server is not involved.

Why. The Atlas is read-only and its server has no encoder; a clip to share should cost the operator one click and no install. The overlay is drawn on the canvas, not the DOM, because only a canvas stream can carry it into the file.

Alternatives rejected.

• ffmpeg.wasm: about 30 MB of wasm for a share button.
• A server-side encoder: the server would need ffmpeg, and the read-only API would gain a rendering job.
• Recording the on-screen canvas as it is: its size and aspect are the window's, not the platform's.
• preserveDrawingBuffer on the renderer, to copy the canvas at any time: it costs every frame of the live view for a feature used rarely. The copy happens in the scene's frame listeners, right after the render.

Consequences.

• While exporting, the scene is in capture mode (beginCapture/endCapture): the renderer and camera take the frame's size and the fit reads that frame, not the container; endCapture must restore the view exactly, whatever ends the export.
• Chromium without a proprietary codec (Playwright's, Firefox) records WebM only, so the browser suite asserts the extension the browser reports and the MP4 choice is covered by the unit test of pickMimeType.
• A hidden tab stops the render loop; the export pauses with it rather than record frozen frames, so the tab must stay visible while it records.
• Node labels are DOM and are not in the clip, by design.