* fix(layers): keep Background semantics across merge, duplication and canvas size
Preserve destination locks in Merge Down, share the normal Background-copy rule with multi-layer duplication, and convert a Background to a paintable layer when extending its canvas transparently.
Add depth-aware regression coverage and ordinary-layer controls for #2380, #2381 and #2384.
* refactor(layers): copy lower-layer locks without cloning Copy data
* Format background_layer_consistency test
---------
Co-authored-by: Brandon Thomas <echelon@gmail.com>
