Update slugify branch audit plan and implement fixes for identified issues

- Marked all todos in the slugify branch audit plan as completed, confirming the successful verification of core invariants and sync processes.
- Introduced a new plan to address critical bugs, correctness issues, and refactors identified during the audit, including enhancements to metadata handling and slugification logic.
- Added tests to ensure proper handling of filenames with forbidden characters and deduplication of instances with colliding names.
- Refactored metadata operations to improve clarity and maintainability, consolidating logic for reading and writing the `name` field in `.meta.json5` files.

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

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

rojo-test/build-tests/tilde_no_meta/src/Foo~1.luau

@@ -0,0 +1,2 @@
+-- Tilde in filename without meta should produce instance name "Foo~1"
+return nil