feat(maps): v0.5.5 — cell_material_slot public getter

Exposes what the v3 vertex/material render path computes internally:
per (layer, x, y), returns {slot, rot, flip} for material cells,
nil for non-material. Override entry wins (normalised via existing
normalize_override_entry); otherwise resolves through
compute_cell_bitmask_v3 + SLOT_LOOKUP.

Primary consumer: debug-overlay tooling (map-editor 0.2.0c.3) that
labels each rendered cell with its blob-14 slot identity for
authoring verification.

manifest.lib

@@ -1 +1 @@
-{"id":"lib-core.maps","version":"0.5.4","api_min":"0.1"}
+{"id":"lib-core.maps","version":"0.5.5","api_min":"0.1"}