feat(map-editor): cell-tile as third state.mode + Tools menu entry

state.set_mode now accepts "cell-tile"; toggle_mode is renamed
cycle_mode (with a backwards-compat alias) and now advances
auto-tile -> cell-tile -> direct -> auto-tile. Tools > Mode
submenu gains a third entry between Auto-Tile and Direct.
Self-test gains t8 (set/get round-trip) and t12 (cycle order).
Startup banner updated to "Tab=cycle mode". Manifest dep pin
bumped to lib-core.maps 0.5.7 to match installed version.

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

manifest.module

@@ -9,7 +9,7 @@
         "blob_rect_stone": "lib-asset.prototype-blob-geom"
     },
     "deps": [
-        {"id":"lib-core.maps","version":"0.5.6"},
+        {"id":"lib-core.maps","version":"0.5.7"},
         {"id":"lib-core.render","version":"0.1.0"},
         {"id":"lib-core.camera","version":"0.3.0"},
         {"id":"lib-core.input","version":"0.4.0"},