style: format the whole tree with cargo fmt (stock defaults, #35)

One-time, mechanical reformat — no functional changes. The tree was not
rustfmt-clean (~1800 hunks across ~100 files); this brings it to stock
`cargo fmt` defaults so a `cargo fmt --check` CI gate can follow.
Behaviour-preserving: 2509 pass / 0 fail / 1 ignored (unchanged baseline),
clippy clean. A .git-blame-ignore-revs entry follows so `git blame`
skips this commit.

tests/it/undo_snapshots.rs

@@ -179,7 +179,10 @@ fn undo_disabled_takes_no_snapshots() {
         .unwrap();
 
         // Nothing to undo, and no snapshot machinery on disk.
-        assert!(db.undo().await.unwrap().is_none(), "undo is a no-op when disabled");
+        assert!(
+            db.undo().await.unwrap().is_none(),
+            "undo is a no-op when disabled"
+        );
         assert!(db.peek_undo().await.unwrap().is_none());
     });
 
@@ -360,7 +363,10 @@ fn undo_restores_db_and_csv_consistently() {
         db.insert(
             "T".to_string(),
             Some(vec!["id".to_string(), "name".to_string()]),
-            vec![Value::Number("1".to_string()), Value::Text("Alice".to_string())],
+            vec![
+                Value::Number("1".to_string()),
+                Value::Text("Alice".to_string()),
+            ],
             Some("insert Alice".to_string()),
         )
         .await
@@ -444,8 +450,8 @@ fn undo_ring_persists_across_reopen() {
     // held by `project`). The persisted ring must survive.
     drop(db);
     let persistence = Persistence::new(path);
-    let db2 = Database::open_with_persistence_and_undo(&db_path, persistence, true)
-        .expect("reopen db");
+    let db2 =
+        Database::open_with_persistence_and_undo(&db_path, persistence, true).expect("reopen db");
 
     rt().block_on(async {
         let peek = db2