feat: record a demo script in a sandbox as a silent MP4 with step timings (#2) - #10

What

flow-reel record demo.ts replays a Playwright demo script in a container (Xvfb + headed Chromium in kiosk mode)
while ffmpeg captures the display, and writes:

• demo.mp4: silent, 1920×1080, 30 fps
• demo.transcript.json: each step's timings (schema v1)
• demo.events.json: the runner's moves, clicks, typing and scrolls (schema v1)
• .flow-reel/demo/master.mp4: the 2560×1440 master for later polish, in a folder that git-ignores itself

The script gets a wrapped page: actions ease the mouse to their target and type visibly, so the polish tickets can
draw the cursor, zooms and click sounds from events.json.

Why

There was no way to record an agent's demo headlessly and reproducibly. Runs are stabilised (fixed viewport, clock,
random seed, reduced motion, a network-idle wait and time budgets per action) so the same script gives the same
video: the same steps, each starting within ±150 ms.

Design: planning/GH-1-narrated-reproducible-feature-demos/ (story decisions 009–013, GH-2 decisions 001–007).

How it was tested

• npm run typecheck, npm test: 36 unit tests (schemas, easing, wrapped page, loader, launcher arguments, network
  idle).
• npm run test:e2e: builds the image and records a fixture app twice. It checks the resolutions and frame count,
  that both JSON files validate, step starts within ±150 ms between runs, and clicks within 2 frames of the video
  frame where they show.
• The README path, from an npm pack install into an empty project.
• On a machine with a load average over ~10, the recording fails with a clear error instead of producing a video
  with wrong timing (decision 007).

Closes #2