feat: harden stale-file detection with mtime+size fingerprint (#17)

Bare mtime equality can miss a concurrent external write that lands
within the filesystem's mtime resolution (same-second writes), or
where the writer restores the original mtime. Add config_fingerprint()
returning a (mtime, size) ConfigStat pair; the stale-file check in
bcc.py now compares both fields instead of mtime alone. config_mtime()
is kept as-is (still used/tested independently).

bcc_core.py

@@ -29,6 +29,7 @@ import threading
 import time
 from dataclasses import dataclass
 from pathlib import Path
+from typing import NamedTuple
 from urllib.parse import urlparse
 
 CONFIG_FILENAME = "claude_desktop_config.json"
@@ -380,6 +381,30 @@ def config_mtime(path: Path | str) -> float | None:
         return None
 
 
+class ConfigStat(NamedTuple):
+    """A snapshot of a config file's mtime + size.
+
+    Pairing size with mtime hardens stale-file detection beyond bare mtime
+    equality: a concurrent external write can land within the filesystem's
+    mtime resolution (e.g. same-second writes on ext4/HFS+) or have its mtime
+    restored by the writing process, in which case mtime alone would miss the
+    change. Comparing both fields catches those cases without the cost of a
+    full content hash.
+    """
+
+    mtime: float
+    size: int
+
+
+def config_fingerprint(path: Path | str) -> ConfigStat | None:
+    """Return the file's (mtime, size) snapshot, or None if it does not exist."""
+    try:
+        st = Path(path).stat()
+    except OSError:
+        return None
+    return ConfigStat(st.st_mtime, st.st_size)
+
+
 def external_change_summary(original_cfg: dict, path: Path | str) -> tuple[list[str], str]:
     """
     Compare original_cfg (what BCC loaded) with the current on-disk state.
@@ -474,7 +499,7 @@ def _normalize_unicode(text: str, notes: list[str]) -> str:
     out = text
     for junk in _JUNK_CHARS:
         out = out.replace(junk, "")
-    out = out.replace(" ", " ")  # non-breaking space
+    out = out.replace(" ", " ")  # non-breaking space
     for smart, ascii_q in _QUOTE_MAP.items():
         out = out.replace(smart, ascii_q)
     if out != text: