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>

scripts/stage-ffmpeg.cjs

@@ -0,0 +1,65 @@
+#!/usr/bin/env node
+
+const { copyFileSync, chmodSync, existsSync, mkdirSync, statSync } = require('node:fs')
+const path = require('node:path')
+const { spawnSync } = require('node:child_process')
+
+const args = process.argv.slice(2)
+const optional = args.includes('--optional')
+const sourceIndex = args.indexOf('--source')
+const explicitSource = sourceIndex >= 0 ? args[sourceIndex + 1] : ''
+
+const root = path.resolve(__dirname, '..')
+const exe = process.platform === 'win32' ? 'ffmpeg.exe' : 'ffmpeg'
+const targetDir = path.join(root, 'bin', process.platform)
+const target = path.join(targetDir, exe)
+
+try {
+  mkdirSync(targetDir, { recursive: true })
+
+  if (existsSync(target) && statSync(target).isFile()) {
+    chmodIfNeeded(target)
+    console.log(`ffmpeg already staged at ${path.relative(root, target)}`)
+    process.exit(0)
+  }
+
+  const source = explicitSource || process.env.FFMPEG_PATH || findOnPath()
+  if (!source) {
+    finishWithoutBinary('ffmpeg was not found. Set FFMPEG_PATH or pass --source /path/to/ffmpeg.')
+  }
+
+  const resolvedSource = path.resolve(source)
+  if (!existsSync(resolvedSource) || !statSync(resolvedSource).isFile()) {
+    finishWithoutBinary(`ffmpeg source is not a file: ${source}`)
+  }
+
+  copyFileSync(resolvedSource, target)
+  chmodIfNeeded(target)
+  console.log(`staged ffmpeg from ${resolvedSource} to ${path.relative(root, target)}`)
+} catch (err) {
+  if (optional) {
+    console.warn(`warning: could not stage ffmpeg: ${err.message}`)
+    process.exit(0)
+  }
+  throw err
+}
+
+function findOnPath() {
+  const command = process.platform === 'win32' ? 'where' : 'which'
+  const result = spawnSync(command, [exe], { encoding: 'utf8' })
+  if (result.status !== 0) return ''
+  return result.stdout.split(/\r?\n/).map((line) => line.trim()).find(Boolean) || ''
+}
+
+function chmodIfNeeded(file) {
+  if (process.platform !== 'win32') chmodSync(file, 0o755)
+}
+
+function finishWithoutBinary(message) {
+  if (optional) {
+    console.warn(`warning: ${message}`)
+    process.exit(0)
+  }
+  console.error(message)
+  process.exit(1)
+}