kernel: regression tests for RmlUi clipping (scissor + stencil clip-mask)

The stage dock's rounded image cards were the first thing to exercise the
substrate's RmlUi clipping path (the slice-3 spike doc never used overflow /
border-radius). Investigated on report of square corners: the renderer's
clipping is CORRECT — added 4 headless+gles2 cases proving it (shm + dmabuf,
asymmetric so a flipped scissor Y would fail):
- overflow:hidden parent clips an oversized child (outside bands transparent);
- border-radius circle clip-mask rounds the corners (corners transparent);
- an image() decorator on a CHILD of a rounded overflow:hidden card clips to the
  rounded shape (the dock's correct structure);
- a transformed rounded clip survives a set_size grow (the dock's real path).

The square-corner bug was RmlUi-core behavior — an element's own image()
decorator is not clipped to its own border-radius (a background-color rounds by
geometry; a decorator needs the clip mask, which self-render never sets) — fixed
in the dock by moving the decorator to a child. No renderer/substrate source
change. kernel 49 cases/208 assertions green on build + build-asan.

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