fix(mod-api): validate active block storage

data/scripts/story3.lua

@@ -913,8 +913,8 @@ M.VERMILION_DOCK = {
       -- the ship is long gone: erase her right away, and anyone who
       -- still lands here is sent back out past the guard unless a mod
       -- explicitly permits this occupied map state.  This hook surrounds
-      -- only the ejection decision; the map's complete onEnter chain has
+      -- only the ejection decision; map-script registration and dispatch
-      -- already run and the departed ship remains erased.
+      -- stay unchanged, and the departed ship remains erased.
       for _, b in ipairs(DOCK_SHIP_BLOCKS) do
         ow.map:setBlock(b.bx, b.by, b.water)
       end

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

src/world/WorldAPI.lua

@@ -146,8 +146,13 @@ function WorldAPI:activeBlockAt(mapId, bx, by)
   if type(def.blocks) ~= "table" or type(map.blockAt) ~= "function" then
     return nil, "block unavailable"
   end
+  local stored = def.blocks[by * def.width + bx + 1]
+  if not validBlockCoordinate(stored) or stored < 0 then
+    return nil, "block unavailable"
+  end
   local ok, blockId = pcall(map.blockAt, map, bx, by)
-  if not ok or not validBlockCoordinate(blockId) or blockId < 0 then
+  if not ok or not validBlockCoordinate(blockId) or blockId < 0
+      or blockId ~= stored then
     return nil, "block unavailable"
   end
   return blockId