A zero-refinement FixedPatchStepper is the PeriodicBlockStepper capability:
one doubly periodic blocked SRT level (no LES, Jets or exchange
observation) whose level_objects[0] are stationary passive circles. The
circle tables come from the same checks, now blocks.static_circle_tables
(moved from block_stepper, pycuda-free); BFL, halo and BlockStep launches
are the shared helpers, with an all-ACTIVE (null) kind. Base-level bodies
stay rejected for depth >= 2. PeriodicBlockStepper and its callers are
unchanged (deleted only after S1's GPU result).
Tests: host contract rejections before CUDA for the single level; GPU
bitwise comparison against PeriodicBlockStepper for every split, with and
without the circle (the S2 gate).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>