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.

src/main.rs

@@ -1,6 +1,6 @@
 use std::process::ExitCode;
 
-use rdbms_playground::cli::{help_text, version_text, Args};
+use rdbms_playground::cli::{Args, help_text, version_text};
 use rdbms_playground::{logging, runtime};
 
 fn main() -> ExitCode {