spec(t27f): export_video -- same display list to MP4 on the lab #10265

Milestone M6 of T27F Live (design: docs/nona-02-organism/t27f/genui-research.md, section 5.3). Export the same display list to video: on the Railway lab, render a recorded log frame by frame to images and encode an MP4 with ffmpeg as plumbing, so the posted clip and the live page are the same frames. The spec fixes frame rate, size presets (1920x1080, 1080x1920) and the per-frame hash record.

Boundary

• specs/t27f/export_video.t27

User Scenarios

• Given a recorded log when it is exported on the lab and replayed in the browser then the per-frame op-stream hashes are equal.
• Given a 30 s log when it is exported at 30 fps then a 900-frame MP4 is produced for an X post.

Requirements

• FR-001: The exporter MUST consume the same display list the browser paints, not a second renderer's scene.
• FR-002: Every exported frame MUST record its op-stream hash.
• FR-003: The encoder MUST run on the Railway lab, never on the owner's Mac.

Success Criteria

• For a 30 s fixture, 900 frames out of 900 have equal hashes between the lab export and the browser replay.
• One 30 s queen-live.mp4 is produced on the lab.