chore(map-editor): bump to v0.3.1; cheatsheet + README sync

Bumps the module version to v0.3.1 (lib-core.maps dep pin was
already bumped to 0.5.7 in CT6 to allow the editor to load).
Updates the file header, startup banner, cheatsheet Tab entry
(now mentions all three modes), and appends a Cell-Tile Mode
subsection to the README.

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

manifest.module

@@ -1,6 +1,6 @@
 {
     "id": "map-editor",
-    "version": "0.3.0",
+    "version": "0.3.1",
     "kind": "module",
     "api": "^0.1",
     "ci_frames": 30,