redbear-btusb: declare scheme mut (handle_sync takes &mut scheme)

main.rs:842 'let scheme = HciScheme::new(...)' but line 868 calls
request.handle_sync(&mut scheme, ...) -> E0596 cannot borrow as mutable.
Missed by --check-sweep because redbear-btusb is a transitive driver dep,
not an explicit config package (sweep only covers config-explicit recipes).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

local/recipes/drivers/redbear-btusb/source/src/main.rs

@@ -839,7 +839,7 @@ fn daemon_main(config: &TransportConfig) -> Result<(), String> {
         }
     };
 
-    let scheme = HciScheme::new(t, controller_info);
+    let mut scheme = HciScheme::new(t, controller_info);
     let socket = Socket::create()
         .map_err(|err| format!("failed to create scheme socket: {err}"))?;
     let mut scheme_state = redox_scheme::scheme::SchemeState::new();