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/esp32s3.rs

@@ -0,0 +1,121 @@
+//! ESP32-S3 MCU initialization primitives.
+//!
+//! Provides low-level bus and peripheral init for ESP32-S3 boards.
+//! Board files call these with specific pins and construct higher-level
+//! drivers (Sx126x, SSD1306, etc.) themselves.
+
+use embassy_sync::blocking_mutex::raw::NoopRawMutex;
+use esp_hal::dma::{DmaRxBuf, DmaTxBuf};
+use esp_hal::i2c::master::{Config as I2cConfig, I2c};
+use esp_hal::spi::master::{Config as SpiConfig, Spi, SpiDmaBus};
+use esp_hal::spi::Mode as SpiMode;
+use esp_hal::time::Rate;
+use esp_hal::timer::timg::TimerGroup;
+use static_cell::StaticCell;
+
+// ── MCU-level types ─────────────────────────────────────────────────
+
+pub type SpiBus = SpiDmaBus<'static, esp_hal::Async>;
+pub type I2cBus = I2c<'static, esp_hal::Async>;
+pub type UsbOtgDriver = esp_hal::otg_fs::asynch::Driver<'static>;
+pub type UartDriver = esp_hal::uart::Uart<'static, esp_hal::Async>;
+
+// ── Timer ───────────────────────────────────────────────────────────
+
+/// Start the ESP-RTOS scheduler and Embassy time driver.
+pub fn start_timer(timg0: esp_hal::peripherals::TIMG0<'static>) {
+    let timg0 = TimerGroup::new(timg0);
+    esp_rtos::start(timg0.timer0);
+}
+
+// ── SPI bus ─────────────────────────────────────────────────────────
+
+/// Initialize SPI2 with DMA and wrap in a shared bus (StaticCell + Mutex).
+///
+/// Returns a reference to the shared SPI bus suitable for `SpiDevice::new()`.
+pub fn init_spi(
+    spi2: esp_hal::peripherals::SPI2<'static>,
+    dma_ch0: esp_hal::peripherals::DMA_CH0<'static>,
+    sck: esp_hal::peripherals::GPIO9<'static>,
+    mosi: esp_hal::peripherals::GPIO10<'static>,
+    miso: esp_hal::peripherals::GPIO11<'static>,
+) -> &'static embassy_sync::mutex::Mutex<NoopRawMutex, SpiBus> {
+    let spi = Spi::new(
+        spi2,
+        SpiConfig::default()
+            .with_frequency(Rate::from_mhz(1))
+            .with_mode(SpiMode::_0),
+    )
+    .expect("SPI init")
+    .with_sck(sck)
+    .with_mosi(mosi)
+    .with_miso(miso)
+    .with_dma(dma_ch0);
+
+    let (rx_buf, rx_desc, tx_buf, tx_desc) = esp_hal::dma_buffers!(256);
+    let dma_rx = DmaRxBuf::new(rx_desc, rx_buf).expect("DMA RX buf");
+    let dma_tx = DmaTxBuf::new(tx_desc, tx_buf).expect("DMA TX buf");
+
+    let spi = spi.with_buffers(dma_rx, dma_tx).into_async();
+
+    static SPI_BUS: StaticCell<embassy_sync::mutex::Mutex<NoopRawMutex, SpiBus>> =
+        StaticCell::new();
+    SPI_BUS.init(embassy_sync::mutex::Mutex::new(spi))
+}
+
+// ── I2C bus ─────────────────────────────────────────────────────────
+
+/// Initialize I2C0 for peripherals (display, sensors, etc.).
+pub fn init_i2c(
+    i2c0: esp_hal::peripherals::I2C0<'static>,
+    sda: esp_hal::peripherals::GPIO17<'static>,
+    scl: esp_hal::peripherals::GPIO18<'static>,
+) -> I2cBus {
+    I2c::new(i2c0, I2cConfig::default())
+        .expect("I2C init")
+        .with_sda(sda)
+        .with_scl(scl)
+        .into_async()
+}
+
+// ── USB OTG ─────────────────────────────────────────────────────────
+
+/// Initialize USB OTG FS driver for CDC-ACM.
+pub fn init_usb(
+    usb0: esp_hal::peripherals::USB0<'static>,
+    dp: esp_hal::peripherals::GPIO20<'static>,
+    dm: esp_hal::peripherals::GPIO19<'static>,
+) -> UsbOtgDriver {
+    let usb_inst = esp_hal::otg_fs::Usb::new(usb0, dp, dm);
+    static EP_OUT_BUF: StaticCell<[u8; 1024]> = StaticCell::new();
+    let ep_out_buf = EP_OUT_BUF.init([0u8; 1024]);
+    esp_hal::otg_fs::asynch::Driver::new(
+        usb_inst,
+        ep_out_buf,
+        esp_hal::otg_fs::asynch::Config::default(),
+    )
+}
+
+// ── UART ────────────────────────────────────────────────────────────
+
+/// Initialize UART0 for boards with USB-UART bridge chips (e.g. CP2102).
+pub fn init_uart(
+    uart0: esp_hal::peripherals::UART0<'static>,
+    tx: esp_hal::peripherals::GPIO43<'static>,
+    rx: esp_hal::peripherals::GPIO44<'static>,
+) -> UartDriver {
+    use esp_hal::uart::{Config as UartConfig, Uart};
+
+    Uart::new(uart0, UartConfig::default())
+        .expect("UART init")
+        .with_tx(tx)
+        .with_rx(rx)
+        .into_async()
+}
+
+// ── MAC address ─────────────────────────────────────────────────────
+
+/// Read the factory-programmed MAC address from eFuse.
+pub fn mac_address() -> [u8; 6] {
+    esp_hal::efuse::Efuse::mac_address()
+}