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/partition/mod.rs

@@ -9,4 +9,4 @@
 
 pub mod plan;
 
-pub use plan::*;
+pub use plan::*;