feat(maps): v0.5.2 — set_vertex / get_vertex public APIs

Vertex-grid write/read for the autotile painting path. The map-editor
0.2.0a Auto-Tile mode is the primary consumer; painting a single
vertex causes up to 4 surrounding cells to flip to material via the
any-corner rule. Coordinate-checks reject OOB writes; grid is lazily
allocated on first paint.

init.lua

@@ -1402,6 +1402,62 @@ function M.get_override(layer_name, x, y, map_id)
     return normalize_override_entry(entry)
 end
 
+-- 0.5.2: vertex-grid write APIs for the autotile painting path.
+-- Vertices live on a (W+1) x (H+1) grid; painting a single vertex
+-- "fills in" the 4 cells around it per the any-corner rule (cell
+-- becomes material). The map-editor's Auto-Tile mode is the
+-- primary consumer.
+
+local function check_vertex_coords(map, vx, vy, fn_name)
+    local vw = map.size.w + 1
+    local vh = map.size.h + 1
+    if vx < 0 or vy < 0 or vx >= vw or vy >= vh then
+        error(string.format("%s: vertex (%d,%d) out of bounds for vertex-grid %dx%d",
+            fn_name, vx, vy, vw, vh))
+    end
+end
+
+local function ensure_vertex_grid(layer, map)
+    if layer.vertices == nil then
+        local vw = map.size.w + 1
+        local vh = map.size.h + 1
+        local grid = {}
+        for i = 1, vw * vh do grid[i] = 0 end
+        layer.vertices = grid
+    end
+    return layer.vertices
+end
+
+function M.set_vertex(layer_name, vx, vy, painted, map_id)
+    local map = require_map(map_id)
+    check_vertex_coords(map, vx, vy, "maps.set_vertex")
+    local layer = require_v3_layer(map, layer_name, "maps.set_vertex")
+    local v = ensure_vertex_grid(layer, map)
+    local vw = map.size.w + 1
+    local idx = vy * vw + vx + 1
+    local new_val = (painted == true or painted == 1) and 1 or 0
+    if v[idx] ~= new_val then
+        v[idx] = new_val
+        map._dirty = true
+        map._opaque_ceiling = nil  -- vertex change may affect opaque ceiling
+        -- _layer_has_content goes true on paint; never auto-cleared
+        -- on erase (stale-true is harmless).
+        if new_val == 1 and map._layer_has_content then
+            map._layer_has_content[layer_name] = true
+        end
+    end
+end
+
+function M.get_vertex(layer_name, vx, vy, map_id)
+    local map = require_map(map_id)
+    check_vertex_coords(map, vx, vy, "maps.get_vertex")
+    local layer = require_v3_layer(map, layer_name, "maps.get_vertex")
+    if layer.vertices == nil then return false end
+    local vw = map.size.w + 1
+    local v = layer.vertices[vy * vw + vx + 1]
+    return v == 1 or v == 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