Add GID encode/decode with range validation

Bit layout [atlas:8][tile_id:20][rot:2][res:2] matches
lib-core.maps init.lua so JS-produced GIDs are byte-identical to
engine-produced ones. >>> 0 forces unsigned u32 output.

src/gid.js

@@ -0,0 +1,30 @@
+'use strict';
+
+const MAX_ATLAS = 255;
+const MAX_TILE = 1048575;
+const MAX_ROT = 3;
+const MAX_U32 = 0xFFFFFFFF;
+
+function checkInt(name, value, min, max) {
+  if (!Number.isInteger(value) || value < min || value > max) {
+    throw new RangeError(`${name} ${value} out of range [${min},${max}]`);
+  }
+}
+
+function encodeGid(atlas, tile, rotation = 0) {
+  checkInt('atlas', atlas, 0, MAX_ATLAS);
+  checkInt('tile', tile, 0, MAX_TILE);
+  checkInt('rotation', rotation, 0, MAX_ROT);
+  return ((atlas << 24) | (tile << 4) | (rotation << 2)) >>> 0;
+}
+
+function decodeGid(gid) {
+  checkInt('gid', gid, 0, MAX_U32);
+  return {
+    atlas: (gid >>> 24) & 0xFF,
+    tile: (gid >>> 4) & 0xFFFFF,
+    rotation: (gid >>> 2) & 0x3,
+  };
+}
+
+module.exports = { encodeGid, decodeGid, MAX_ATLAS, MAX_TILE, MAX_ROT };