chore(map-editor): bump to v0.3.0; cheatsheet + README sync

Updates the in-editor Cheatsheet to reflect the new G binding and
the Esc cascade; bumps manifest.module and the file header to
v0.3.0; adds a Menu UI section to README.md.

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

manifest.module

@@ -1,6 +1,6 @@
 {
     "id": "map-editor",
-    "version": "0.2.0c.4",
+    "version": "0.3.0",
     "kind": "module",
     "api": "^0.1",
     "ci_frames": 30,