fix: address PR #596 review issues (#597)

## Summary

Fixes three issues identified in the [post-merge review](https://github.com/heygen-com/hyperframes/pull/596#pullrequestreview-4214283515) of PR #596:

- **P1 (cache bypass):** When `extractCacheDir` is set, extracted frames live outside `compiledDir`, so `createCompiledFrameSrcResolver` rejects them and every frame falls back to base64 data URIs. Fix: symlink cached frame directories into `compiledDir/__hyperframes_video_frames/` after extraction and remap `framePaths` so the served-frame fast path works.

- **P2 (pooled browser stale state):** `closeCaptureSession` force-killed the Chrome process on timeout via raw `SIGKILL` without clearing `pooledBrowser` / `pooledBrowserRefCount`, leaving other sessions with a dead browser reference. Fix: add `forceReleaseBrowser()` in `browserManager` that atomically clears pool state before killing the process.

- **P3 (reserved chars in URLs):** `createCompiledFrameSrcResolver` encodes path segments with `encodeURIComponent`, but the file server used `c.req.path` (which only applies `decodeURI`) to look up files on disk. Video IDs containing `#`, `?`, or `%` produced 404s. Fix: apply `decodeURIComponent` per path segment in the file server's catch-all route.

## Test plan

- [x] `createCompiledFrameSrcResolver` tests: symlinked cache paths resolve to served URLs; cache-external paths return null; reserved characters encode correctly
- [x] `forceReleaseBrowser` tests: kills process + disconnects; tolerates already-killed process
- [x] `createFileServer` test: `video%231/frame.jpg` serves file from `video#1/frame.jpg` on disk
- [x] Typecheck: engine + producer pass
- [x] Lint + format: 0 warnings, 0 errors

packages/engine/src/services/browserManager.test.ts

@@ -1,6 +1,6 @@
-import { describe, expect, it } from "vitest";
+import { describe, expect, it, vi } from "vitest";
 
-import { buildChromeArgs } from "./browserManager.js";
+import { buildChromeArgs, forceReleaseBrowser } from "./browserManager.js";
 
 describe("buildChromeArgs browser GPU mode", () => {
   const base = { width: 1920, height: 1080 };
@@ -46,3 +46,33 @@ describe("buildChromeArgs browser GPU mode", () => {
     expect(args).not.toContain("--use-angle=metal");
   });
 });
+
+describe("forceReleaseBrowser", () => {
+  it("kills the browser process and disconnects", () => {
+    const killFn = vi.fn(() => true);
+    const disconnectFn = vi.fn();
+    const mockBrowser = {
+      process: () => ({ kill: killFn, killed: false }),
+      disconnect: disconnectFn,
+    } as any;
+
+    forceReleaseBrowser(mockBrowser);
+
+    expect(killFn).toHaveBeenCalledWith("SIGKILL");
+    expect(disconnectFn).toHaveBeenCalled();
+  });
+
+  it("tolerates an already-killed process", () => {
+    const killFn = vi.fn();
+    const disconnectFn = vi.fn();
+    const mockBrowser = {
+      process: () => ({ kill: killFn, killed: true }),
+      disconnect: disconnectFn,
+    } as any;
+
+    forceReleaseBrowser(mockBrowser);
+
+    expect(killFn).not.toHaveBeenCalled();
+    expect(disconnectFn).toHaveBeenCalled();
+  });
+});