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>

src/GameEngine.h

@@ -169,7 +169,7 @@ public:
     int getFrame() { return currentFrame; }
     float getFrameTime() { return frameTime; }
     sf::View getView() { return visible; }
-    void manageTimer(std::string, PyObject*, int);
+    // Note: manageTimer() removed in #173 - use Timer objects directly
     std::shared_ptr<Timer> getTimer(const std::string& name);
     void setWindowScale(float);
     bool isHeadless() const { return headless; }