Proves that previously-unreachable atlas slots become reachable
via the cell-tile material path: slot 0 isolated (t3), slot 1 end
with correct rotations (t4), slot 4 straight (t5), slot 5
tee_open (t6), slot 8 cross_open (t7).
Upgrades write_test.map.json to schema_version 3 with material
and vertices fields on surface so cell_material_slot + set_vertex
resolve correctly in the tests.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
