mount: prefer boot disk ESP and run cargo fmt

* choose ESP matching the primary data disk when multiple ESPs exist,
  falling back gracefully for single-disk layouts
* keep new helper to normalize device names and reuse the idempotent
  mount logic
* apply cargo fmt across the tree

src/errors.rs

@@ -53,4 +53,4 @@ pub enum Error {
 }
 
 /// Crate-wide result alias.
-pub type Result<T> = std::result::Result<T, Error>;
+pub type Result<T> = std::result::Result<T, Error>;