fix: re-anchor effects clock when ticker drains (stale-clock freeze)

After Ticker.stop(), elapsed resets to zero on the next start(). _now was
left frozen at the old elapsed, so effects added after a drain captured a
stale start time and their progress() clamped to 0 forever — ticker never
stopped, second batch broken. Fix: reset _now = Duration.zero on drain.
Adds @visibleForTesting getters and a regression test that catches the
stale value directly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/ui/widgets/effects_overlay.dart

@@ -60,11 +60,22 @@ class EffectsOverlayState extends State<EffectsOverlay>
     _ticker = createTicker(_tick);
   }
 
+  @visibleForTesting
+  Ticker get ticker => _ticker;
+
+  @visibleForTesting
+  Duration get now => _now;
+
   void _tick(Duration elapsed) {
     setState(() {
       _now = elapsed;
       _fx.removeWhere((e) => e.done(elapsed));
-      if (_fx.isEmpty) _ticker.stop();
+      if (_fx.isEmpty) {
+        _ticker.stop();
+        // Ticker elapsed restarts from zero after stop(); re-anchor so
+        // effects added later don't inherit a stale clock.
+        _now = Duration.zero;
+      }
     });
   }