feat(tools): add screencast recording engine

Adds tools/screencast/ — a reusable screen-recording pipeline for
skill demo videos. Wraps a pytest desktop E2E run with:
  - ScreenRecorder: mss-based full-screen frame capture (background thread)
  - SubtitleEngine: stdout keyword → timestamped SRT generation
  - compose_video: FFmpeg filter_complex merge (video + subtitles + BGM)
  - run_screencast: main orchestrator wiring all three layers

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tools/screencast/recorder.py

@@ -0,0 +1,48 @@
+"""全屏截帧引擎，后台线程持续截图存为 PNG 帧序列。"""
+from __future__ import annotations
+
+import threading
+import time
+from pathlib import Path
+
+import mss
+import mss.tools
+
+
+class ScreenRecorder:
+    def __init__(self, frames_dir: str, fps: int = 10):
+        self._frames_dir = Path(frames_dir)
+        self._fps = fps
+        self._stop_event = threading.Event()
+        self._thread: threading.Thread | None = None
+        self._frame_count = 0
+
+    def start(self) -> None:
+        self._frames_dir.mkdir(parents=True, exist_ok=True)
+        self._stop_event.clear()
+        self._frame_count = 0
+        self._thread = threading.Thread(target=self._capture_loop, daemon=True)
+        self._thread.start()
+
+    def stop(self) -> None:
+        self._stop_event.set()
+        if self._thread:
+            self._thread.join(timeout=10)
+
+    @property
+    def frame_count(self) -> int:
+        return self._frame_count
+
+    def _capture_loop(self) -> None:
+        interval = 1.0 / self._fps
+        with mss.mss() as sct:
+            monitor = sct.monitors[0]  # 全屏（monitor[0] 是所有显示器合并区域）
+            while not self._stop_event.is_set():
+                t0 = time.perf_counter()
+                img = sct.grab(monitor)
+                path = self._frames_dir / f"frame_{self._frame_count:06d}.png"
+                mss.tools.to_png(img.rgb, img.size, output=str(path))
+                self._frame_count += 1
+                elapsed = time.perf_counter() - t0
+                sleep = max(0.0, interval - elapsed)
+                time.sleep(sleep)