Add set_roof write API to lib-core.maps

Allocates the roof array on-demand if the map didn't have one,
validates value in {0, 1}, and bounds-checks the target cell.

init.lua

@@ -680,6 +680,25 @@ function M.set_cell_gid(layer_name, x, y, gid, map_id)
     map._dirty = true
 end
 
+function M.set_roof(x, y, value, map_id)
+    local map = require_map(map_id)
+    if x < 0 or y < 0 or x >= map.size.w or y >= map.size.h then
+        error(string.format("maps.set_roof: cell (%d,%d) out of bounds for size %dx%d",
+            x, y, map.size.w, map.size.h))
+    end
+    if value ~= 0 and value ~= 1 then
+        error(string.format("maps.set_roof: value %s must be 0 or 1", tostring(value)))
+    end
+    if not map.roof then
+        local cell_count = map.size.w * map.size.h
+        map.roof = {}
+        for i = 1, cell_count do map.roof[i] = 0 end
+    end
+    local idx = y * map.size.w + x + 1
+    map.roof[idx] = value
+    map._dirty = true
+end
+
 function M.is_indoor(x, y, map_id)
     local id = map_id or current_map_id
     if not id then error("maps.is_indoor: no current map") end