feat: auto-resolve init-name conflicts during syncback

When a child instance has a Roblox name that would produce a filesystem
name of "init" (case-insensitive), syncback now automatically prefixes
it with '_' (e.g. "Init" → "_Init.luau") instead of erroring. The
corresponding meta.json writes the original name via the `name` property
so Rojo can restore it on the next snapshot.

The sibling dedup check is updated to use actual on-disk names for
existing children and the resolved (init-prefixed) name for new ones,
so genuine collisions still error while false positives from the `name`
property are avoided.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/syncback/snapshot.rs

@@ -237,6 +237,24 @@ pub fn inst_path(dom: &WeakDom, referent: Ref) -> String {
     path.join("/")
 }
 
+impl<'sync> SyncbackData<'sync> {
+    /// Constructs a `SyncbackData` for use in unit tests.
+    #[cfg(test)]
+    pub fn for_test(
+        vfs: &'sync Vfs,
+        old_tree: &'sync RojoTree,
+        new_tree: &'sync WeakDom,
+        project: &'sync Project,
+    ) -> Self {
+        Self {
+            vfs,
+            old_tree,
+            new_tree,
+            project,
+        }
+    }
+}
+
 #[cfg(test)]
 mod test {
     use rbx_dom_weak::{InstanceBuilder, WeakDom};