Document v0.2.0 API and schema-v2 map format

Update README.md to version 0.2.0: add API entries for the 16 new
public functions (encode_gid, decode_gid, upgrade_v1_to_v2,
validate_map_table_v2, atlas_count, atlas_id_at, has_layer, is_indoor,
cell_gid, tile_at_layer, blocks_walk, blocks_sight,
iterate_layers_pre_entities, iterate_layers_post_entities,
draw_map_pre_entities, draw_map_post_entities), add the schema-v2
reference section documenting layer-stack, packed-u32 GID bit layout,
auto-migration, and a v0.2.0 changelog entry.

Fix tile-id OOB validation regression in build_map_v2: after v1->v2
upgrade, packed GIDs were not checked against atlas palette sizes.
Restore the check in build_map_v2 so loading a map with an out-of-range
tile_id still produces a fatal error with the same message pattern.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

init.lua

@@ -275,6 +275,25 @@ end
 
 local function build_map_v2(t_map, resolved_atlases)
     local size = t_map.size
+    -- Validate that each non-zero GID references a tile_id within its atlas's palette.
+    local layers = t_map.layers or {}
+    for layer_name, layer_data in pairs(layers) do
+        if VALID_LAYER_NAMES[layer_name] and layer_data.tiles then
+            for i, gid in ipairs(layer_data.tiles) do
+                if gid ~= 0 then
+                    local atlas_idx, tile_id, _rot = decode_gid(gid)
+                    local atlas = resolved_atlases[atlas_idx + 1]
+                    if atlas then
+                        if tile_id < 1 or tile_id > #atlas.tiles then
+                            error(string.format(
+                                "maps.load: tile-id %d at index %d exceeds palette size %d (in map '%s', layer '%s')",
+                                tile_id, i, #atlas.tiles, t_map.id or "<unnamed>", layer_name))
+                        end
+                    end
+                end
+            end
+        end
+    end
     return {
         id              = t_map.id,
         schema_version  = 2,
@@ -282,7 +301,7 @@ local function build_map_v2(t_map, resolved_atlases)
         tile_size       = resolved_atlases[1].tile_size,  -- assume uniform; first atlas wins
         atlases         = resolved_atlases,
         atlas_aliases   = t_map.atlases,  -- alias strings, parallel to atlases
-        layers          = t_map.layers or {},
+        layers          = layers,
         roof            = t_map.roof,
         -- DEPRECATED-MVP stubs
         walls   = {},