refactor: extract hal/ and driver/ modules

- Created hal/esp32s3.rs with MCU primitives (SPI, I2C, USB, UART, timer, MAC)
- Created hal/nrf52840.rs with shared nRF boilerplate (interrupts, bus wrapping, VBUS, MAC)
- Moved SH1106 OLED driver from display/ to driver/ (clear separation from app code)
- Board esp32s3.rs now delegates MCU init to hal, keeps only peripheral wiring (RadioParts)
- nRF board files use hal for shared StaticCell patterns, mac_address, interrupt bindings
- Board files still own pin assignments and peripheral construction

src/main.rs

@@ -21,6 +21,10 @@ mod board;
 mod channel;
 #[cfg(not(test))]
 mod display;
+#[cfg(not(test))]
+mod driver;
+#[cfg(not(test))]
+mod hal;
 mod protocol;
 #[cfg(not(test))]
 mod radio;