Add audit plan for slugify branch pass 3 and implement critical fixes

- Introduced a comprehensive audit plan for the slugify branch, detailing findings from a holistic review of 110 files, including 1 critical bug and 2 correctness issues.
- Completed todos addressing model JSON renaming, syncback updates, and test enhancements for roundtrip fidelity.
- Added tests for reserved names and ensured correct handling of slugified instances, preserving compound extensions in model files.
- Refactored change processing logic to correctly manage the `name` field in model files, enhancing metadata handling and ensuring compliance with round-trip identity standards.

src/syncback/file_names.rs

@@ -323,7 +323,9 @@ pub fn validate_file_name<S: AsRef<str>>(name: S) -> anyhow::Result<()> {
 
 /// Known script suffixes that appear between the base name and file extension.
 /// For example, in `MyScript.server.luau`, `.server` is the suffix.
-const KNOWN_SCRIPT_SUFFIXES: &[&str] = &[".server", ".client", ".plugin", ".local", ".legacy"];
+const KNOWN_SCRIPT_SUFFIXES: &[&str] = &[
+    ".server", ".client", ".plugin", ".local", ".legacy", ".model",
+];
 
 /// Strips a known script suffix from a file stem.
 ///