test(rhythmruler): add comprehensive unit tests for RhythmRuler widget

- Added js/widgets/rhythmruler.test.js with ~40 tests coverage:
  - Constructor & initialization
  - State management & undo functionality
  - Note width calculation (validates layout scaling)
  - Rhythm manipulation (dissect, save history)
  - Playback state tracking
- Added module.exports to rhythmruler.js for Jest compatibility
- Used proper mocking pattern for window/document globals

Author: WillyEverGreen

js/widgets/rhythmruler.js

@@ -2846,3 +2846,6 @@ class RhythmRuler {
         }
     }
 }
+if (typeof module !== "undefined") {
+    module.exports = RhythmRuler;
+}