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/composer.py

@@ -0,0 +1,71 @@
+"""FFmpeg 合成：帧序列 + SRT 字幕 + 背景音乐 → MP4。"""
+from __future__ import annotations
+
+import random
+import subprocess
+import sys
+from pathlib import Path
+
+
+def compose_video(
+    frames_dir: str,
+    fps: int,
+    subtitle_path: str,
+    music_dir: str,
+    output_path: str,
+    music_volume: float = 0.15,
+) -> str:
+    frames_dir = Path(frames_dir)
+    subtitle_path = Path(subtitle_path)
+    output_path = Path(output_path)
+    output_path.parent.mkdir(parents=True, exist_ok=True)
+
+    # 随机选一首背景音乐
+    music_files = list(Path(music_dir).rglob("*.mp3"))
+    if not music_files:
+        raise FileNotFoundError(f"music_dir 下没有找到 MP3 文件: {music_dir}")
+    music_file = random.choice(music_files)
+    print(f"[screencast] 背景音乐: {music_file.name}")
+
+    # FFmpeg 字幕路径在 Windows 下需要转义冒号（subtitles 滤镜内部语法）
+    srt_path_str = str(subtitle_path).replace("\\", "/").replace(":", "\\:")
+
+    subtitle_filter = (
+        f"subtitles='{srt_path_str}'"
+        ":force_style='FontName=Arial,FontSize=28,"
+        "PrimaryColour=&H00FFFFFF,OutlineColour=&H00000000,"
+        "Outline=2,Shadow=1,Alignment=2'"
+    )
+
+    # 视频和音频都放进 filter_complex，避免 -vf 与 -filter_complex 混用报错
+    filter_complex = (
+        f"[0:v]{subtitle_filter}[vout];"
+        f"[1:a]volume={music_volume},apad[aout]"
+    )
+
+    cmd = [
+        "ffmpeg", "-y",
+        "-framerate", str(fps),
+        "-i", str(frames_dir / "frame_%06d.png"),
+        "-i", str(music_file),
+        "-filter_complex", filter_complex,
+        "-map", "[vout]",
+        "-map", "[aout]",
+        "-c:v", "libx264",
+        "-preset", "fast",
+        "-crf", "23",
+        "-c:a", "aac",
+        "-b:a", "128k",
+        "-shortest",
+        "-pix_fmt", "yuv420p",
+        str(output_path),
+    ]
+
+    print(f"[screencast] 运行 FFmpeg 合成...")
+    result = subprocess.run(cmd, capture_output=True, text=True, encoding="utf-8", errors="replace")
+    if result.returncode != 0:
+        print(result.stderr, file=sys.stderr)
+        raise RuntimeError(f"FFmpeg 合成失败，退出码 {result.returncode}")
+
+    print(f"[screencast] 输出: {output_path}")
+    return str(output_path)