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/help_command.rs

@@ -11,7 +11,7 @@
 use crossterm::event::{KeyCode, KeyEvent, KeyEventKind, KeyModifiers};
 
 use rdbms_playground::app::App;
-use rdbms_playground::dsl::{parse_command, AppCommand, Command};
+use rdbms_playground::dsl::{AppCommand, Command, parse_command};
 use rdbms_playground::event::AppEvent;
 
 const fn key(code: KeyCode) -> AppEvent {