Support object extension indexers

notes/object_indexer_extension_optimization.md

@@ -0,0 +1,18 @@
+# Object indexer extension follow-up
+
+Context
+- `Obj.getAt` / `Obj.putAt` now support scoped extension indexers for named singleton `object` declarations, so patterns like `Storage["name"]` can route through `override fun Storage.getAt(...)` and `override fun Storage.putAt(...)`.
+- Dispatch order is intentionally:
+  1. concrete non-root class members
+  2. scoped extensions
+  3. built-in string-field fallback
+
+Why this order
+- Normal class-defined indexers are the common fast path and should not pay extension lookup cost first.
+- Checking scoped extensions earlier caused avoidable overhead for regular overridden indexers.
+- Routing extension indexers through the generic root `Obj.getAt` member path caused recursion; direct extension dispatch avoids that.
+
+Possible future optimization
+- If extension-based indexers show up in hot profiles, add a tiny receiver-shape/member cache at the extension-dispatch points in `Obj.getAt` / `Obj.putAt`.
+- The cache should stay behind the concrete non-root member fast path.
+- A good shape key is likely `(receiver class id/layout version, member name)` with the cached extension record/wrapper.