Add map inspector that emits a human-readable stats report

Counts set vs empty cells per layer, lists the top-10 most frequent
non-empty GIDs with decoded components, and reports roof coverage.
When an atlas registry is supplied, atlas-id+tile-name annotations
are added; without one the report still works using indices only.

tests/fixtures/atlases-inspector/atlas_a/tiles.atlas.json

@@ -0,0 +1,10 @@
+{
+  "atlas_id": "atlas_a",
+  "atlas_version": 1,
+  "atlas_size_px": [64, 64],
+  "tile_size_px": 32,
+  "tiles": [
+    { "id": 1, "name": "grass", "uv": [0, 0, 32, 32], "walkable": true },
+    { "id": 2, "name": "stone", "uv": [32, 0, 32, 32], "walkable": false, "blocks_sight": true }
+  ]
+}