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/hal/mod.rs

@@ -0,0 +1,10 @@
+//! MCU family initialization primitives.
+//!
+//! Each submodule provides low-level bus and peripheral init for one MCU family.
+//! Board files call these with specific pins and construct higher-level drivers.
+
+#[cfg(any(feature = "heltec_v3", feature = "heltec_v4"))]
+pub mod esp32s3;
+
+#[cfg(any(feature = "rak_wisblock_4631", feature = "wio_tracker_l1"))]
+pub mod nrf52840;