feat: add v1->v2 schema auto-upgrade for map tables

Converts legacy single-tilemap maps (tiles[], tile_rotations[]) into the
schema-v2 format (atlases[], layers.surface with packed GID cells).

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

init.lua

@@ -43,6 +43,40 @@ local function decode_gid(gid)
     return atlas_index, tile_id, rotation
 end
 
+-- =====================================================================
+-- Schema v1→v2 Auto-Upgrade (transparent beim Load)
+-- v1: { id, tilemap, size, tiles[], tile_rotations? }
+-- v2: { schema_version, id, size, atlases[], layers: { surface: { tiles[] } }, roof? }
+-- =====================================================================
+
+local function upgrade_v1_to_v2(v1)
+    local atlas_id = v1.tilemap or "default_tilemap"
+    local v1_tiles = v1.tiles or {}
+    local v1_rots  = v1.tile_rotations or {}
+
+    local gids = {}
+    for i = 1, #v1_tiles do
+        local tile_id = v1_tiles[i]
+        local rot_deg = v1_rots[i] or 0
+        local rot_quad = math.floor(rot_deg / 90) % 4
+        if tile_id == 0 then
+            gids[i] = 0
+        else
+            gids[i] = encode_gid(0, tile_id, rot_quad)
+        end
+    end
+
+    return {
+        schema_version = 2,
+        id   = v1.id,
+        size = v1.size,
+        atlases = { atlas_id },
+        layers = {
+            surface = { tiles = gids }
+        }
+    }
+end
+
 -- =====================================================================
 -- Internal helpers
 -- =====================================================================
@@ -354,5 +388,6 @@ end
 
 M.encode_gid = encode_gid
 M.decode_gid = decode_gid
+M.upgrade_v1_to_v2 = upgrade_v1_to_v2
 
 return M