Restores the PeriodicBlockStepper safeguard dropped in 8e1feff, for all
FixedPatchStepper levels: both G buffers, halo pairs and boundary columns
of every level are compared with cuda.mem_get_info() before the stream,
any module or any buffer exists, raising MemoryError('level buffers exceed
free device memory'). Transfer tables are not counted (lower bound).
Tests: the single-level MemoryError case is back beside the param-layout
check (no stream, no allocation); host test_single_level covers depth 1
and depth 2 (no stream, build or allocation) and the wait failure after a
good step (count stays 1). Driver-mocking fixtures report ample free memory.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
