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/iteration5_export_import.rs

@@ -14,8 +14,8 @@ use crossterm::event::{KeyCode, KeyEvent, KeyEventKind, KeyModifiers};
 use rdbms_playground::action::Action;
 use rdbms_playground::app::App;
 use rdbms_playground::archive::{
-    default_export_filename, export_project, extract_into, inspect_zip,
-    next_export_sequence, resolve_import_target,
+    default_export_filename, export_project, extract_into, inspect_zip, next_export_sequence,
+    resolve_import_target,
 };
 use rdbms_playground::event::AppEvent;
 use rdbms_playground::project::{HISTORY_LOG, PLAYGROUND_DB, PROJECT_YAML};
@@ -295,11 +295,9 @@ fn end_to_end_export_then_import_real_project() {
     // Build a populated source project.
     let src_path = {
         let p = project::Project::create_named(&data.path().join("Source")).unwrap();
-        let db = Database::open_with_persistence(
-            p.db_path(),
-            Persistence::new(p.path().to_path_buf()),
-        )
-        .unwrap();
+        let db =
+            Database::open_with_persistence(p.db_path(), Persistence::new(p.path().to_path_buf()))
+                .unwrap();
         rt().block_on(async {
             db.create_table(
                 "Customers".to_string(),
@@ -362,7 +360,11 @@ fn end_to_end_export_then_import_real_project() {
 
     // Round-trip: the inserted row is back.
     let data_view = rt()
-        .block_on(async { imported_db.query_data("Customers".to_string(), None, None).await })
+        .block_on(async {
+            imported_db
+                .query_data("Customers".to_string(), None, None)
+                .await
+        })
         .expect("query data");
     assert_eq!(data_view.rows.len(), 1);
     // Serial id auto-filled to 1; Name was the inserted value.