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/maps/inspect_sample.map.json

@@ -0,0 +1,30 @@
+{
+  "schema_version": 2,
+  "id": "inspect_sample",
+  "size": { "w": 4, "h": 4 },
+  "atlases": ["atlas_a", "atlas_b"],
+  "layers": {
+    "surface": {
+      "tiles": [
+        16, 16, 16, 16,
+        16, 32, 32, 16,
+        16, 32, 32, 16,
+        16, 16, 16, 16
+      ]
+    },
+    "wall": {
+      "tiles": [
+        16777232, 16777232, 16777232, 16777232,
+        16777232,        0,        0, 16777232,
+        16777232,        0,        0, 16777232,
+        16777232, 16777232, 16777232, 16777232
+      ]
+    }
+  },
+  "roof": [
+    0, 0, 0, 0,
+    0, 1, 1, 0,
+    0, 1, 1, 0,
+    0, 0, 0, 0
+  ]
+}