Add desktop/server MP4 export pipeline, hardening, and render tests (v1.3.0)

Desktop MP4 export renders frames from a hidden BrowserWindow via
capturePage, pipes PNGs to a bundled ffmpeg (libx264/yuv420p/+faststart),
and writes the file through an atomic temp-file swap. It shares scene
validation (shared/renderValidation.cjs) and the render.html renderer with
the standalone Puppeteer server, which is hardened in parallel.

Render worker & main (electron/):
- abortable render loop with an inactivity watchdog and cancel plumbing
- atomic temp-file + rename so a cancel/failure never truncates a
  pre-existing target (#7); stdin EPIPE / early-exit / backpressure-hang
  handled (#8)
- HiDPI frames normalized to canvas size (#10); odd dimensions padded to
  even (#13, also server); backgroundThrottling disabled (#12)
- reject canvases larger than the display (#11); single-render guard;
  isolated render session that blocks network requests (#5)

Tests:
- Electron harnesses (scripts/verify-*.cjs) for capture, export, cancel,
  timeout, assets, background, capture-fidelity, resolution, odd-dims,
  aggregated by verify-desktop-render.cjs
- server smoke + e2e and shared validation unit tests
- headless hook (src/devtools/headless.ts) to run the in-app suite under
  browser/electron hosts; pixel-diff util + capture fixtures (test/)
- dev_docs plans: integration, fixes (#7-#13), and the e2e/capture strategy

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

bin/README.md

@@ -0,0 +1,16 @@
+# ffmpeg release binaries
+
+Desktop MP4 export looks for ffmpeg at:
+
+- `bin/linux/ffmpeg`
+- `bin/darwin/ffmpeg`
+- `bin/win32/ffmpeg.exe`
+
+These binaries are packaged into Electron resources as `resources/bin/<platform>/`.
+Use `npm run stage:ffmpeg -- --source /path/to/ffmpeg` to copy a local binary
+into the correct folder, or set `FFMPEG_PATH` when running the app to use an
+external ffmpeg without staging it.
+
+Release builds should use a redistributable static ffmpeg build for each target
+platform. Record the source URL, version, configured codecs, and license terms
+for every binary in this folder before publishing an installer.