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/ffmpegBinaries.test.ts

@@ -0,0 +1,43 @@
+// fallow-ignore-file code-duplication
+import { resolve } from "node:path";
+import { afterEach, describe, expect, it } from "vitest";
+import {
+  assertConfiguredFfmpegBinariesExist,
+  getFfmpegBinary,
+  getFfprobeBinary,
+} from "./ffmpegBinaries.js";
+
+describe("ffmpeg binary env resolution", () => {
+  const originalFfmpegPath = process.env.HYPERFRAMES_FFMPEG_PATH;
+  const originalFfprobePath = process.env.HYPERFRAMES_FFPROBE_PATH;
+
+  afterEach(() => {
+    if (originalFfmpegPath === undefined) delete process.env.HYPERFRAMES_FFMPEG_PATH;
+    else process.env.HYPERFRAMES_FFMPEG_PATH = originalFfmpegPath;
+    if (originalFfprobePath === undefined) delete process.env.HYPERFRAMES_FFPROBE_PATH;
+    else process.env.HYPERFRAMES_FFPROBE_PATH = originalFfprobePath;
+  });
+
+  it("uses configured absolute paths when env vars are set", () => {
+    process.env.HYPERFRAMES_FFMPEG_PATH = "/tools/ffmpeg.exe";
+    process.env.HYPERFRAMES_FFPROBE_PATH = "/tools/ffprobe.exe";
+
+    expect(getFfmpegBinary()).toBe(resolve("/tools/ffmpeg.exe"));
+    expect(getFfprobeBinary()).toBe(resolve("/tools/ffprobe.exe"));
+  });
+
+  it("throws a clear error when a configured FFmpeg path is missing", () => {
+    process.env.HYPERFRAMES_FFMPEG_PATH = "/missing/ffmpeg.exe";
+
+    expect(() => assertConfiguredFfmpegBinariesExist()).toThrow(
+      /FFmpeg binary not found at HYPERFRAMES_FFMPEG_PATH/,
+    );
+  });
+
+  it("accepts existing configured paths", () => {
+    process.env.HYPERFRAMES_FFMPEG_PATH = process.execPath;
+    process.env.HYPERFRAMES_FFPROBE_PATH = process.execPath;
+
+    expect(() => assertConfiguredFfmpegBinariesExist()).not.toThrow();
+  });
+});