Timer refactor: stopwatch-like semantics, mcrfpy.timers collection closes #173

Major Timer API improvements:
- Add `stopped` flag to Timer C++ class for proper state management
- Add `start()` method to restart stopped timers (preserves callback)
- Add `stop()` method that removes from engine but preserves callback
- Make `active` property read-write (True=start/resume, False=pause)
- Add `start=True` init parameter to create timers in stopped state
- Add `mcrfpy.timers` module-level collection (tuple of active timers)
- One-shot timers now set stopped=true instead of clearing callback
- Remove deprecated `setTimer()` and `delTimer()` module functions

Timer callbacks now receive (timer, runtime) instead of just (runtime).
Updated all tests to use new Timer API and callback signature.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/unit/test_step_function.py

@@ -63,13 +63,13 @@ def run_tests():
     print("Test 5: Timer fires after step() advances past interval")
     timer_fired = [False]  # Use list for mutable closure
 
-    def on_timer(runtime):
-        """Timer callback - receives runtime in ms"""
+    def on_timer(timer, runtime):
+        """Timer callback - receives timer object and runtime in ms"""
         timer_fired[0] = True
         print(f"    Timer fired at simulation time={runtime}ms")
 
     # Set a timer for 500ms
-    mcrfpy.setTimer("test_timer", on_timer, 500)
+    test_timer = mcrfpy.Timer("test_timer", on_timer, 500)
 
     # Step 600ms - timer should fire (500ms interval + some buffer)
     dt = mcrfpy.step(0.6)
@@ -88,7 +88,7 @@ def run_tests():
         print("  Skipping timer test in windowed mode")
 
     # Clean up
-    mcrfpy.delTimer("test_timer")
+    test_timer.stop()
     print()
 
     # Test 6: Error handling - invalid argument type