Remove MRT from the FixedPatch2D rejection matrix (TRT and other
rejections retained); convert host MRT tests to stdlib unittest; add GPU
gates for unshifted dense-vs-blocked MRT two-level roots (exact bits) and
a shifted all-blocked three-level MRT smoke checking per-level omega.
No numerical source change.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>