UniformCollection accessor methods checked the raw collection pointer
but never checked if the owning object was still alive. Changed owner
field from weak_ptr<UIDrawable> to weak_ptr<void> (type-erased) so
both UIDrawable and UIEntity owners can be tracked. Set owner in both
get_uniforms() paths. All accessors now check owner.lock() before
dereferencing the raw collection pointer.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
