fix(daemon): gate NullSnapshotStore behind cfg(test) — silence release dead_code warning

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/spaceshd/src/snapshot_store.rs

@@ -17,8 +17,11 @@ pub enum SnapshotMsg {
     Remove(SurfaceId),
 }
 
-/// A no-op store for tests and contexts that do not persist snapshots.
+/// A no-op store for tests that do not persist snapshots. Test-only — gated so
+/// release builds don't warn about an unconstructed struct.
+#[cfg(test)]
 pub struct NullSnapshotStore;
+#[cfg(test)]
 impl SnapshotStore for NullSnapshotStore {
     fn save(&self, _sid: &SurfaceId, _snap: &Snapshot) {}
     fn load(&self, _sid: &SurfaceId) -> Option<Snapshot> { None }