Fix --git-since live sync not detecting changes and creating duplicates

Two issues prevented --git-since from working correctly during live sync:

1. Server: File changes weren't detected because git-filtered project nodes
   had empty relevant_paths, so the change processor couldn't map VFS events
   back to tree instances. Fixed by registering $path directories and the
   project folder in relevant_paths even when filtered.

2. Plugin: When a previously-filtered file was first acknowledged, it appeared
   as an ADD patch. The plugin created a new instance instead of adopting the
   existing one in Studio, causing duplicates. Fixed by checking for untracked
   children with matching Name+ClassName before calling Instance.new.

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

src/snapshot_middleware/project.rs

@@ -83,6 +83,19 @@ pub fn snapshot_project(
             // file being updated.
             snapshot.metadata.relevant_paths.push(path.to_path_buf());
 
+            // When git filter is active, also register the project folder as a
+            // relevant path. This serves as a catch-all so that file changes
+            // not under any specific $path node can still walk up the directory
+            // tree and trigger a re-snapshot of the entire project.
+            if context.has_git_filter() {
+                if let Some(folder) = path.parent() {
+                    let normalized = vfs
+                        .canonicalize(folder)
+                        .unwrap_or_else(|_| folder.to_path_buf());
+                    snapshot.metadata.relevant_paths.push(normalized);
+                }
+            }
+
             Ok(Some(snapshot))
         }
         None => Ok(None),
@@ -137,6 +150,16 @@ pub fn snapshot_project_node(
             // Take the snapshot's metadata as-is, which will be mutated later
             // on.
             metadata = snapshot.metadata;
+        } else if context.has_git_filter() {
+            // When the git filter is active and the $path was filtered out
+            // (no acknowledged files yet), we still need to register the path
+            // in relevant_paths. This allows the change processor to map file
+            // changes in this directory back to this project node instance,
+            // triggering a re-snapshot that will pick up newly modified files.
+            let normalized = vfs
+                .canonicalize(full_path.as_ref())
+                .unwrap_or_else(|_| full_path.to_path_buf());
+            metadata.relevant_paths.push(normalized);
         }
     }