FixedPatchStepper(..., child=(x0,y0,x1,y1), child_block=(by,bx)) adds an L2
patch in L1 window cells (requires fine_block/base_block, one L1 patch, no
bodies/jets). nest_patch() builds the L1->L2 FixedPatch2D (2*nu on L1) and
the composed L1 kind (inherited INNER/OUTER kept, child-covered ACTIVE made
OUTER), rejecting from the actual tables: coverage outside ACTIVE, donors or
receivers not uncovered ACTIVE, surviving ACTIVE pulls of covered/OUTER cells
without a child-supplied arrival, and overlap with L1's restriction footprint.
Step order per L1 hop: explode L1(src)->L2, two L2 hops (halo, step), L1 halo,
receiver-local L2->L1 staging into src, L1 step. The second hop thus feeds L2
from the new L1 G; L2 is never refilled from L0; L0 stays once per step (1:2:4).
Existing kernels only; a small _Pair holds the child transfer bindings (explode
from the child module, parent window stride). Two-level API unchanged.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
