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.ts

@@ -0,0 +1,63 @@
+// fallow-ignore-file code-duplication
+import { execFileSync } from "child_process";
+import { existsSync } from "fs";
+import { resolve } from "path";
+
+export const FFMPEG_PATH_ENV = "HYPERFRAMES_FFMPEG_PATH";
+export const FFPROBE_PATH_ENV = "HYPERFRAMES_FFPROBE_PATH";
+
+const pathCache = new Map<string, string | undefined>();
+
+function findOnPath(name: "ffmpeg" | "ffprobe"): string | undefined {
+  if (pathCache.has(name)) return pathCache.get(name);
+  try {
+    const command = process.platform === "win32" ? "where" : "which";
+    const output = execFileSync(command, [name], {
+      encoding: "utf-8",
+      stdio: ["pipe", "pipe", "pipe"],
+      timeout: 5000,
+    });
+    const first = output
+      .split(/\r?\n/)
+      .map((s) => s.trim())
+      .find(Boolean);
+    const resolved = first ? resolve(first) : undefined;
+    pathCache.set(name, resolved);
+    return resolved;
+  } catch {
+    pathCache.set(name, undefined);
+    return undefined;
+  }
+}
+
+function getConfiguredBinary(envName: string, binaryName: "ffmpeg" | "ffprobe"): string {
+  const configured = process.env[envName]?.trim();
+  if (configured) return resolve(configured);
+  return findOnPath(binaryName) ?? binaryName;
+}
+
+export function getFfmpegBinary(): string {
+  return getConfiguredBinary(FFMPEG_PATH_ENV, "ffmpeg");
+}
+
+export function getFfprobeBinary(): string {
+  return getConfiguredBinary(FFPROBE_PATH_ENV, "ffprobe");
+}
+
+export function assertConfiguredFfmpegBinariesExist(): void {
+  const ffmpegPath = process.env[FFMPEG_PATH_ENV]?.trim();
+  if (ffmpegPath && !existsSync(ffmpegPath)) {
+    throw new Error(
+      `[FFmpeg] FFmpeg binary not found at ${FFMPEG_PATH_ENV}="${ffmpegPath}". ` +
+        "Install FFmpeg or unset the override.",
+    );
+  }
+
+  const ffprobePath = process.env[FFPROBE_PATH_ENV]?.trim();
+  if (ffprobePath && !existsSync(ffprobePath)) {
+    throw new Error(
+      `[FFmpeg] FFprobe binary not found at ${FFPROBE_PATH_ENV}="${ffprobePath}". ` +
+        "Install FFmpeg or unset the override.",
+    );
+  }
+}