Add lock-file load and stable ID assignment

Lock-file determinism guarantees re-bakes do not renumber existing
tiles. Atlas_id mismatch is a hard-fail to prevent silent corruption.
Deleted tile IDs move to deleted[] and are not recycled.

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

tests/fixtures/lock-existing.json

@@ -0,0 +1,9 @@
+{
+  "atlas_id": "small_demo",
+  "bindings": {
+    "grass": 1,
+    "stone": 2
+  },
+  "deleted": [],
+  "next_id": 3
+}