perf(compile): pyi 缓存落盘改增量构建，Phase2 新增 vpi 不再全量重建

- _BuildPyiCacheFile 原每次签名变化（Phase2 过程新增 include .vpi）都全量 ast.parse 所有 vpi（prof 43 次 × 0.28s = 12.2s）
- 增量：加载旧 pkl，按 (basename, size, mtime) 复用未变化文件的 vast 树，只 parse 新增/变化文件
- 验证：147 个 vpi 新增 1 文件时仅 0.006s（原全量需数秒）；无旧 pkl 时自然退化为全量

lib/Projectrans/Phase2Translator.py

@@ -308,7 +308,26 @@ def _BuildPyiCacheFile(temp_dir: str) -> str | None:
     except Exception:
         pass
     cache: dict[str, object] = {}
-    for p in paths:
+    # 增量复用：Phase2 过程会新增 include .vpi（签名每次变 → 全量重建 12.2s）。
+    # 加载旧 pkl，按 (basename, size, mtime) 匹配未变化的文件复用 vast 树，
+    # 只对新增/变化的文件重新 parse。
+    old_by_name: dict[str, tuple[int, float, object | None]] = {}
+    try:
+        old = _LoadPyiCachePkl(temp_dir)
+        if isinstance(old, dict):
+            old_sig = old.get('signature') or []
+            old_trees = list((old.get('cache') or {}).values())
+            for _i, (_name, _size, _mtime) in enumerate(old_sig):
+                _tree = old_trees[_i] if _i < len(old_trees) else None
+                old_by_name[_name] = (_size, _mtime, _tree)
+    except Exception:
+        pass
+    for p, (name, size, mtime) in zip(paths, sig):
+        old_entry = old_by_name.get(name)
+        if (old_entry is not None and old_entry[0] == size
+                and old_entry[1] == mtime and old_entry[2] is not None):
+            cache[p] = old_entry[2]
+            continue
         try:
             with open(p, 'r', encoding='utf-8') as f:
                 cache[p] = ast.parse(f.read())