Add 42 new assertions covering: bw-compat register without opts,
multi-active open(A)+open(B) leaving both open, per-id close(id),
bw-compat close()/is_open() operating on the focused panel, layout-
slot resolution (left-half / right-half / center default / custom-fn),
Loud-Error on unknown slot name, hit-test dispatch against window
bounds with reverse open-order, is_pausing scanning all open windows.
Existing 12 baseline assertions retained and still pass via the bw-
compat shim. Total 54 pass / 0 fail.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
