feat: resolve duplicate sibling names with incrementing suffixes

Instead of bailing when children have duplicate filesystem names,
syncback now resolves collisions by appending incrementing suffixes
(e.g. Foo, Foo1, Foo2). This handles both init-renamed children and
any other name collisions. Meta stem derivation is now path-based
to correctly handle collision suffixes and dotted names.

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

src/syncback/mod.rs

@@ -28,7 +28,7 @@ use crate::{
     Project,
 };
 
-pub use file_names::{extension_for_middleware, name_for_inst, slugify_name, validate_file_name};
+pub use file_names::{extension_for_middleware, name_for_inst, validate_file_name};
 pub use fs_snapshot::FsSnapshot;
 pub use hash::*;
 pub use property_filter::{filter_properties, filter_properties_preallocated};

src/syncback/snapshot.rs

@@ -31,6 +31,25 @@ pub struct SyncbackSnapshot<'sync> {
 }
 
 impl<'sync> SyncbackSnapshot<'sync> {
+    /// Computes the middleware and filesystem name for a child without
+    /// creating a full snapshot. Uses the same logic as `with_joined_path`.
+    pub fn child_middleware_and_name(
+        &self,
+        new_ref: Ref,
+        old_ref: Option<Ref>,
+    ) -> anyhow::Result<(Middleware, String)> {
+        let temp = Self {
+            data: self.data,
+            old: old_ref,
+            new: new_ref,
+            path: PathBuf::new(),
+            middleware: None,
+        };
+        let middleware = get_best_middleware(&temp, self.data.force_json);
+        let name = name_for_inst(middleware, temp.new_inst(), temp.old_inst())?;
+        Ok((middleware, name.into_owned()))
+    }
+
     /// Constructs a SyncbackSnapshot from the provided refs
     /// while inheriting this snapshot's path and data. This should be used for
     /// directories.
@@ -251,6 +270,7 @@ impl<'sync> SyncbackData<'sync> {
             old_tree,
             new_tree,
             project,
+            force_json: false,
         }
     }
 }