refactor: flatten radio.rs and display.rs, zero warnings

- Flattened radio/ directory to radio.rs (was just one file)
- Flattened display/ to display.rs (combined task + rendering as inline module)
- Moved SH1106 display driver reference from display/ to driver/ (done in Phase 2)
- Suppressed dead_code warnings for conditional USB/UART types in hal
- All boards: zero warnings, zero cfg in application code

src/board/esp32s3.rs

@@ -15,6 +15,7 @@ use crate::hal::esp32s3 as mcu;
 
 // ── Re-export MCU types ─────────────────────────────────────────────
 
+#[allow(unused_imports)] // Each board uses one of USB/UART, not both
 pub use mcu::{I2cBus as DisplayI2c, SpiBus, UartDriver, UsbOtgDriver as UsbDriver};
 
 // ── Concrete peripheral types ───────────────────────────────────────