fix(mod-api): validate active block storage

docs/modding.md

@@ -247,6 +247,9 @@ or malformed active block returns `nil, "block unavailable"`. The caller must
 require every expected cell to match before changing presentation. This method
 is Gen 1-only; Gold callers receive no parity promise for it.
 
+The same unavailable result covers missing or sparse active block storage and
+an accessor result that does not match its validated active block slot.
+
 ### Conditional map occupancy
 
 `map.occupancy_allowed` is a narrow Gen 1 hook around a map script's vanilla

docs/rfcs/0013-map-occupancy-and-active-block.md

@@ -89,6 +89,10 @@ Failure reasons are stable:
 | Coordinate is negative or outside the active map | `block coordinates out of bounds` |
 | Active block data is absent or malformed | `block unavailable` |
 
+The block slot and the active map accessor must both contain the same valid
+nonnegative integer. Missing or sparse storage and inconsistent accessor data
+return `block unavailable`.
+
 Requiring the expected map ID and rejecting all ambiguous input lets a mod
 compare every cell in its version-specific signature before it calls an
 existing mutation API. Red, Blue, and Yellow each use their own loaded map