masks and symbol selection

Timeline mask source layers do not contribute to the enclosing q0rg's selection/transform bounds or recursive body hit testing. The masked content uses its full original geometry and media bounds, without intersecting them with the mask. This applies recursively to nested symbols and to both cached and uncached placement bounds.

Mask source artwork remains editable through its own layer when editing that symbol. Existing layer visibility/locking and vector erase/fragment geometry rules remain applicable. The rendering, masking, export and project format paths are unchanged.

A release q0editor build includes this change. Native UI interaction was not verified in this turn; no tests were added or run.