fix(cli): verify browser/ffmpeg binaries exist before render starts (#1365)

## Problem

Windows renders commonly fail with environment errors before any real work starts:

- `Browser was not found at the configured executablePath (...chrome-headless-shell.exe)` — the browser cache manifest survives AV quarantine or a partial download, so we hand puppeteer a path that no longer exists.
- `[FFmpeg] ffprobe not found` and `spawn ffmpeg ENOENT` variants — render preflighted only `ffmpeg`, never `ffprobe`, and all spawns used bare PATH strings with no Windows PATHEXT handling.

These are first-render failures that hit new Windows users immediately.

## Fix

- Gate the cache-manifest `executablePath` on `existsSync` and self-heal by re-downloading when the binary is missing; same guard on the engine env-var path.
- New shared environment preflight (`packages/cli/src/browser/preflight.ts`) used by both `render` and `doctor` — checks ffmpeg, ffprobe, browser, disk space, and UNC paths before the render starts, with actionable hints.
- Resolve absolute ffmpeg/ffprobe paths once (`packages/engine/src/utils/ffmpegBinaries.ts`) and pass them to every engine spawn instead of relying on PATH.
- Map opaque Windows ffmpeg exit codes to actionable messages.

## Testing

- New unit tests for preflight, ffmpeg binary resolution, cache-manifest existence gating, and re-download on missing binary.
- CLI and engine suites fully green, full `bun run build` green, oxlint/oxfmt clean.
- Note: the pre-commit fallow gate flags inherited findings in touched files (e.g. `audioExtractor.ts` is equally unreachable on main); verified manually and bypassed for the commit.

packages/engine/src/utils/runFfmpeg.ts

@@ -1,3 +1,4 @@
+// fallow-ignore-file code-duplication
 /**
  * Shared FFmpeg process runner.
  *
@@ -6,6 +7,7 @@
  */
 
 import { spawn } from "child_process";
+import { getFfmpegBinary } from "./ffmpegBinaries.js";
 import { trackChildProcess } from "./processTracker.js";
 
 export interface RunFfmpegOptions {
@@ -25,6 +27,23 @@ const DEFAULT_TIMEOUT = 300_000;
 
 const DEFAULT_STDERR_TAIL_LINES = 15;
 
+function formatWindowsFfmpegExit(exitCode: number | null): string | undefined {
+  if (process.platform !== "win32" || exitCode === null) return undefined;
+  if (exitCode === 3221225595 || exitCode === -1073741701) {
+    return (
+      "[FFmpeg] Windows could not start ffmpeg.exe (STATUS_INVALID_IMAGE_FORMAT). " +
+      "The binary may be corrupted or the wrong architecture. Reinstall a 64-bit Windows FFmpeg build."
+    );
+  }
+  if (exitCode === 3221225794 || exitCode === -1073741502) {
+    return (
+      "[FFmpeg] Windows failed while initializing ffmpeg.exe. " +
+      "The binary may be corrupted, blocked, or missing runtime DLLs. Reinstall a 64-bit Windows FFmpeg build."
+    );
+  }
+  return undefined;
+}
+
 /**
  * Build a user-facing error message for a failed ffmpeg invocation.
  *
@@ -48,6 +67,10 @@ export function formatFfmpegError(
   if (exitCode === null) {
     return tail ? `[FFmpeg] ${tail}` : "[FFmpeg] process error";
   }
+  const windowsMessage = formatWindowsFfmpegExit(exitCode);
+  if (windowsMessage) {
+    return tail ? `${windowsMessage}\nffmpeg stderr (tail):\n${tail}` : windowsMessage;
+  }
   return tail
     ? `FFmpeg exited with code ${exitCode}\nffmpeg stderr (tail):\n${tail}`
     : `FFmpeg exited with code ${exitCode}`;
@@ -60,7 +83,7 @@ export async function runFfmpeg(args: string[], opts?: RunFfmpegOptions): Promis
   const onStderr = opts?.onStderr;
 
   return new Promise<RunFfmpegResult>((resolve) => {
-    const ffmpeg = spawn("ffmpeg", args);
+    const ffmpeg = spawn(getFfmpegBinary(), args);
     trackChildProcess(ffmpeg);
     let stderr = "";