Implement fixes and enhancements from audit pass 2

- Introduced a new plan addressing critical bugs, correctness issues, and refactors identified during the audit, including improvements to metadata handling and slugification logic.
- Replaced inline removal logic in `handle_api_write` with a new `remove_instance_at_path` method to ensure correct deletion of instances.
- Updated pre-seeding logic in `dir.rs` and `project.rs` to derive deduplication keys from actual filesystem paths instead of slugified names.
- Enhanced rename handling in `change_processor.rs` to prevent collisions by deduplicating names against existing siblings.
- Added tests for various scenarios, including handling of forbidden characters and ensuring correct deduplication behavior.
- Refactored child iteration in `dir.rs` and `project.rs` to sort children alphabetically for deterministic deduplication ordering.

rojo-test/build-tests/model_json_name_override/default.project.json5

@@ -0,0 +1,6 @@
+{
+  "name": "model_json_name_override",
+  "tree": {
+    "$path": "src"
+  }
+}