fix(maps): v0.5.6 — derive bitmask from cell's own corner paint-tiles

compute_cell_bitmask_v3 now reads the 8-bit neighbour bitmask from
the render-cell's own 4 corner paint-tiles (cardinal bit set iff
at least one of the 2 paint-tiles on that edge is painted; diagonal
bit set iff the corner paint-tile is painted), not from the
any-corner material status of the 8 neighbour render-cells.

Pre-0.5.6 used cell-neighbour-material, which let two cells share
connectivity across an empty paint-tile gap whenever any unrelated
corner of either was painted — producing connected blob shapes
where two visually separated 2x2 islands were expected.

Atlas, paint storage, override sublayer, public API and the slot
lookup table all unchanged. Doc comment and README painting-model
section rewritten to describe the dual-grid offset explicitly.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

manifest.lib

@@ -1 +1 @@
-{"id":"lib-core.maps","version":"0.5.5","api_min":"0.1"}
+{"id":"lib-core.maps","version":"0.5.6","api_min":"0.1"}