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/undo.rs

@@ -737,7 +737,11 @@ mod tests {
         let payload_dirs = fs::read_dir(&store.root)
             .unwrap()
             .filter_map(std::result::Result::ok)
-            .filter(|e| e.file_name().to_str().is_some_and(|n| n.parse::<u64>().is_ok()))
+            .filter(|e| {
+                e.file_name()
+                    .to_str()
+                    .is_some_and(|n| n.parse::<u64>().is_ok())
+            })
             .count();
         assert_eq!(payload_dirs, 2, "ring capped at 2 payloads");
     }
@@ -766,7 +770,11 @@ mod tests {
         let payload_dirs = fs::read_dir(&store.root)
             .unwrap()
             .filter_map(std::result::Result::ok)
-            .filter(|e| e.file_name().to_str().is_some_and(|n| n.parse::<u64>().is_ok()))
+            .filter(|e| {
+                e.file_name()
+                    .to_str()
+                    .is_some_and(|n| n.parse::<u64>().is_ok())
+            })
             .count();
         assert_eq!(payload_dirs, 1, "only the surviving undo payload remains");
     }
@@ -820,6 +828,10 @@ mod tests {
         fs::create_dir_all(store.payload_dir(41)).unwrap();
         stage_finalize(&store, &fx.conn, "cmd");
         let meta = store.peek_undo().unwrap().unwrap();
-        assert!(meta.id >= 42, "id allocated above the orphan, got {}", meta.id);
+        assert!(
+            meta.id >= 42,
+            "id allocated above the orphan, got {}",
+            meta.id
+        );
     }
 }