fix: address PR #296 code review feedback

- Clamp ML predictions between physics floor (raw airtime) and ceiling
  (worst-case formula) so model can never produce unsafe timeouts
- Replace hourOfDay feature with secondsSinceLastRx for network activity
- Remove unused _ContactStats.stdDev and dead model persistence code
- Debounce observation writes (2s) instead of writing on every delivery
- Skip recording observations when pathLength is null to avoid corrupting
  training data
- Add comment explaining global (not per-contact) RX time tracking
- Remove notifyListeners from retrain to avoid unnecessary widget rebuilds
- Run dart format

test/services/timeout_prediction_service_test.dart

@@ -64,9 +64,9 @@ void main() {
     expect(direct4!, greaterThan(direct2!));
     expect(direct2, greaterThan(direct0!));
 
-    // All should be within the clamp range
-    expect(direct0, greaterThanOrEqualTo(2000));
-    expect(direct4, lessThanOrEqualTo(120000));
+    // All should be positive
+    expect(direct0, greaterThan(0));
+    expect(direct4, greaterThan(0));
 
     // Print predictions for visibility
     debugPrint('Predictions (with 1.5x safety margin):');