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/engine_vocabulary_audit.rs

@@ -32,10 +32,8 @@ use rdbms_playground::event::AppEvent;
 
 const FORBIDDEN: &[&str] = &[
     // Product names.
-    "SQLite", "sqlite",
-    // Crate name.
-    "rusqlite",
-    // Engine-specific keywords / idioms.
+    "SQLite", "sqlite",   // Crate name.
+    "rusqlite", // Engine-specific keywords / idioms.
     "STRICT", "PRAGMA",
 ];
 
@@ -52,9 +50,7 @@ fn engine_vocab_leak(s: &str) -> Option<(&'static str, usize)> {
 
 fn assert_clean(label: &str, s: &str) {
     if let Some((needle, pos)) = engine_vocab_leak(s) {
-        panic!(
-            "ADR-0002 leak in {label}: found `{needle}` at byte {pos} in:\n{s}"
-        );
+        panic!("ADR-0002 leak in {label}: found `{needle}` at byte {pos} in:\n{s}");
     }
 }
 
@@ -118,8 +114,7 @@ fn parse_errors_use_no_engine_vocabulary() {
         "this is not a command",
     ];
     for input in inputs {
-        let err = parse_command(input)
-            .expect_err(&format!("expected parse failure for `{input}`"));
+        let err = parse_command(input).expect_err(&format!("expected parse failure for `{input}`"));
         let rendered = format!("{err:?}");
         assert_clean(&format!("parse error for `{input}`"), &rendered);
     }