feat: Migrate to Python 3.14 (closes #135)

Replace deprecated Python C API calls with modern PyConfig-based initialization:
- PySys_SetArgvEx() -> PyConfig.argv (deprecated since 3.11)
- Py_InspectFlag -> PyConfig.inspect (deprecated since 3.12)

Fix critical memory safety bugs discovered during migration:
- PyColor::from_arg() and PyVector::from_arg() now return new references
  instead of borrowed references, preventing use-after-free when callers
  call Py_DECREF on the result
- GameEngine::testTimers() now holds a local shared_ptr copy during
  callback execution, preventing use-after-free when timer callbacks
  call delTimer() on themselves

Fix double script execution bug with --exec flag:
- Scripts were running twice because GameEngine constructor executed them,
  then main.cpp deleted and recreated the engine
- Now reuses existing engine and just sets auto_exit_after_exec flag

Update test syntax to use keyword arguments for Frame/Caption constructors.

Test results: 127/130 passing (97.7%)

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

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

src/GameEngine.cpp

@@ -336,9 +336,14 @@ void GameEngine::testTimers()
     auto it = timers.begin();
     while (it != timers.end())
     {
-        it->second->test(now);
-        
-        // Remove timers that have been cancelled or are one-shot and fired
+        // Keep a local copy of the timer to prevent use-after-free.
+        // If the callback calls delTimer(), the map entry gets replaced,
+        // but we need the Timer object to survive until test() returns.
+        auto timer = it->second;
+        timer->test(now);
+
+        // Remove timers that have been cancelled or are one-shot and fired.
+        // Note: Check it->second (current map value) in case callback replaced it.
         if (!it->second->getCallback() || it->second->getCallback() == Py_None)
         {
             it = timers.erase(it);