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/subtitle.py

@@ -0,0 +1,55 @@
+"""字幕引擎：根据 stdout 关键词打时间戳，生成 SRT 字幕文件。"""
+from __future__ import annotations
+
+import time
+from dataclasses import dataclass
+from pathlib import Path
+from typing import List, Tuple
+
+
+@dataclass
+class _Entry:
+    start: float   # 距录制开始的秒数
+    text: str
+    duration: float = 5.0  # 每条字幕默认显示 5 秒
+
+
+class SubtitleEngine:
+    def __init__(self, script: List[Tuple[str, str]], default_duration: float = 5.0):
+        """
+        script: [(关键词, 字幕文案), ...]
+        关键词大小写不敏感，每个关键词只触发一次。
+        """
+        self._script = script
+        self._default_duration = default_duration
+        self._entries: List[_Entry] = []
+        self._matched: set[str] = set()
+        self._t0: float = 0.0
+
+    def set_start_time(self, t: float) -> None:
+        self._t0 = t
+
+    def process_line(self, line: str) -> None:
+        lower = line.lower()
+        for keyword, text in self._script:
+            if keyword in self._matched:
+                continue
+            if keyword.lower() in lower:
+                elapsed = time.time() - self._t0
+                self._entries.append(_Entry(start=elapsed, text=text, duration=self._default_duration))
+                self._matched.add(keyword)
+
+    def generate_srt(self, output_path: str) -> None:
+        def _fmt(s: float) -> str:
+            h = int(s // 3600)
+            m = int((s % 3600) // 60)
+            sec = int(s % 60)
+            ms = int((s % 1) * 1000)
+            return f"{h:02d}:{m:02d}:{sec:02d},{ms:03d}"
+
+        Path(output_path).parent.mkdir(parents=True, exist_ok=True)
+        with open(output_path, "w", encoding="utf-8") as f:
+            for i, e in enumerate(self._entries, 1):
+                f.write(f"{i}\n")
+                f.write(f"{_fmt(e.start)} --> {_fmt(e.start + e.duration)}\n")
+                f.write(f"{e.text}\n\n")