feat: use mtime+size fingerprint for stale-file check (#17)

MainWindow._loaded_mtime -> _loaded_stat (core.ConfigStat), populated
via core.config_fingerprint() at load/save time. The save-path stale
check now compares the full fingerprint (mtime AND size) instead of
bare mtime equality.

bcc.py

@@ -1104,7 +1104,7 @@ class MainWindow(QMainWindow):
         self.full_config: dict = {}
         self.servers: list[core.ServerEntry] = []
         self.current_profile: core.Profile | None = None
-        self._loaded_mtime: float | None = None
+        self._loaded_stat: core.ConfigStat | None = None
         self.dirty = False
         self._suppress_table = False
         self._suppress_sel = False
@@ -1380,7 +1380,7 @@ class MainWindow(QMainWindow):
                 return
             self.full_config = cfg
             repaired = True
-        self._loaded_mtime = core.config_mtime(profile.path)
+        self._loaded_stat = core.config_fingerprint(profile.path)
         self.current_profile = profile
         self.servers = core.extract_servers(self.full_config)
         self.dirty = False
@@ -1719,11 +1719,14 @@ class MainWindow(QMainWindow):
             return
 
         # Stale-file check: if the file changed on disk since we loaded it, prompt.
-        disk_mtime = core.config_mtime(self.current_profile.path)
+        # Compare mtime AND size (not mtime alone) so a concurrent external write
+        # that lands within the mtime resolution window, or that restores the
+        # original mtime, still gets caught.
+        disk_stat = core.config_fingerprint(self.current_profile.path)
         if (
-            disk_mtime is not None
-            and self._loaded_mtime is not None
-            and disk_mtime != self._loaded_mtime
+            disk_stat is not None
+            and self._loaded_stat is not None
+            and disk_stat != self._loaded_stat
         ):
             changed_keys, server_diff = core.external_change_summary(
                 self.full_config, self.current_profile.path
@@ -1744,7 +1747,7 @@ class MainWindow(QMainWindow):
                     QMessageBox.critical(self, "Save failed", str(e))
                     return
                 self.full_config = fresh
-                self._loaded_mtime = core.config_mtime(self.current_profile.path)
+                self._loaded_stat = core.config_fingerprint(self.current_profile.path)
                 self.current_profile.config_exists = True
                 self.dirty = False
                 self.save_btn.setEnabled(False)
@@ -1762,7 +1765,7 @@ class MainWindow(QMainWindow):
         except Exception as e:
             QMessageBox.critical(self, "Save failed", str(e))
             return
-        self._loaded_mtime = core.config_mtime(self.current_profile.path)
+        self._loaded_stat = core.config_fingerprint(self.current_profile.path)
         self.current_profile.config_exists = True
         self.dirty = False
         self.save_btn.setEnabled(False)