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>

electron/ffmpegPath.cts

@@ -0,0 +1,30 @@
+import { app } from 'electron'
+import fs from 'node:fs'
+import path from 'node:path'
+
+export function resolveFfmpegPath(): string {
+  const envPath = process.env.FFMPEG_PATH?.trim()
+  if (envPath) return envPath
+
+  const exe = process.platform === 'win32' ? 'ffmpeg.exe' : 'ffmpeg'
+  const bundledPath = path.join(process.resourcesPath, 'bin', process.platform, exe)
+  if (isUsableBinary(bundledPath)) return bundledPath
+
+  return 'ffmpeg'
+}
+
+export function missingFfmpegMessage(): string {
+  if (app.isPackaged) {
+    return 'ffmpeg is not bundled with this RUWIO build, or it cannot be executed. Install ffmpeg or set FFMPEG_PATH to export MP4.'
+  }
+  return 'ffmpeg was not found. Install ffmpeg or set FFMPEG_PATH to export MP4.'
+}
+
+function isUsableBinary(file: string): boolean {
+  try {
+    fs.accessSync(file, process.platform === 'win32' ? fs.constants.F_OK : fs.constants.X_OK)
+    return true
+  } catch {
+    return false
+  }
+}