prepare_fine_boundary accepts a FixedPatchSet2D: objects are projected once
onto the level-wide fine grid (FixedPatchSet2D.fine_origin = (0, 0), same
coarse cell-edge -> fine cell transform), each circle must lie inside one
patch, coarse transfer cells are checked against the covered union, and body
ids stay object positions, so the existing jet one-body-per-jet, overlap and
(receiver, dir) writer checks run once over the whole level (caller obj_id
cannot bypass them). The stepper no longer rejects fine_objects/jets with
several patches; blocked link tables, BFL and the jet amplitude cache are
reused unchanged. No kernel change. README/docstring scope updated.
Tests: CPU translation/ids/rejections (outside one patch, one jet over two
bodies at zero amplitude); GPU one body via the new path == single-patch path
(both modes), two bodies (+jets, amplitude switch) == tiled single (bitwise).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
