docs: M5d-2 tiers given test keeps TempDir as _dir

Setup.dir was only held so the directory outlived Store; clippy
dead_code failed the task 10 gate. The field is now _dir.

Co-Authored-By: Grok 4.6 <noreply@x.ai>

docs/plans/M5d2/files/crates/memoryd/tests/tiers.rs

@@ -13,7 +13,7 @@ fn now() -> Timestamp {
 }
 
 struct Setup {
-    dir: TempDir,
+    _dir: TempDir,
     store: Store,
 }
 
@@ -22,7 +22,7 @@ fn setup(tag: &str) -> Setup {
     let starter = dir.write("etc/soul.md", "Soul.\n");
     let store = Store::new(&dir.path().join("memory"));
     store.init(&starter).unwrap();
-    Setup { dir, store }
+    Setup { _dir: dir, store }
 }
 
 fn write_recall(store: &Store, line: &str) {