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-test-snapshots/end_to_end__tests__build__tilde_no_meta.snap

@@ -0,0 +1,19 @@
+---
+source: tests/tests/build.rs
+expression: contents
+---
+<roblox version="4">
+  <Item class="Folder" referent="0">
+    <Properties>
+      <string name="Name">tilde_no_meta</string>
+    </Properties>
+    <Item class="ModuleScript" referent="1">
+      <Properties>
+        <string name="Name">Foo~1</string>
+        <string name="Source"><![CDATA[-- Tilde in filename without meta should produce instance name "Foo~1"
+return nil
+]]></string>
+      </Properties>
+    </Item>
+  </Item>
+</roblox>

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