refactor(src): file_cache.py Phase 11.3.5 - extract _get_mtime_safe

Phase 11.3.5. The original try/except (OSError, ValueError): mtime = 0.0
in get_cached_tree is now extracted to a Result-returning helper.

The helper returns Result[float]; the caller uses .data (0.0 fallback) and
can inspect .errors. The convention requires Result[T] for try/except sites
that can fail; the helper satisfies this requirement.

Audit post-migration:
- _get_mtime_safe L48 = INTERNAL_COMPLIANT (Heuristic A) ✓
- get_cached_tree L92 = no try/except for mtime (extracted)

Tests: 24/24 pass (test_ast_parser, test_file_cache_no_top_level_tree_sitter).

src/file_cache.py

@@ -40,9 +40,33 @@ import re
 from pathlib import Path
 from typing  import Any, Dict, List, Optional, Tuple
 
+from src.result_types import ErrorInfo, ErrorKind, Result
+
 
 _ast_cache: Dict[str, Tuple[float, tree_sitter.Tree]] = {}
 
+def _get_mtime_safe(path: Optional[str]) -> Result[float]:
+ """Get file mtime, returning Result[float] with errors on OSError/ValueError.
+
+ The convention requires Result[T] for try/except sites that can fail. Used
+ by ASTParser.get_cached_tree to abstract the mtime computation; the caller
+ uses `.data` (0.0 fallback) and can inspect `.errors` if needed.
+ """
+ if path is None:
+  return Result(data=0.0)
+ try:
+  p = Path(path)
+  mtime = p.stat().st_mtime if p.exists() else 0.0
+  return Result(data=mtime)
+ except (OSError, ValueError) as e:
+  return Result(data=0.0, errors=[ErrorInfo(
+   kind=ErrorKind.INTERNAL,
+   message=f"failed to get mtime for {path}: {e}",
+   source="file_cache._get_mtime_safe",
+   original=e,
+  )])
+
+
 class ASTParser:
  """
  Parser for extracting AST-based views of source code.
@@ -78,11 +102,8 @@ class ASTParser:
   if not path:
    return self.parse(code)
   
-  try:
-   p     = Path(path)
-   mtime = p.stat().st_mtime if p.exists() else 0.0
-  except (OSError, ValueError):
-   mtime = 0.0
+  mtime_result = _get_mtime_safe(path)
+  mtime = mtime_result.data  # 0.0 on error (Result.errors has the details)
 
   if path in _ast_cache:
    cached_mtime, tree = _ast_cache[path]