closes#251
Two related bugs where Python garbage collection destroyed callbacks
that were still needed by live C++ objects:
1. **Drawable callbacks (all 8 types)**: tp_dealloc unconditionally called
click_unregister() etc., destroying callbacks even when the C++ object
was still alive in a parent's children vector. Fixed by guarding with
shared_ptr::use_count() <= 1 — only unregister when the Python wrapper
is the last owner.
2. **Timer GC prevention**: Active timers now hold a Py_INCREF'd reference
to their Python wrapper (Timer::py_wrapper), preventing GC while the
timer is registered in the engine. Released on stop(), one-shot fire,
or destruction. mcrfpy.Timer("name", cb, 100) now works without storing
the return value.
Also includes audio synth demo UI fixes: button click handling (don't set
on_click on Caption children), single-column slider layout, improved
Animalese contrast.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
