feat: show MAC address on splash screen

Splash/waiting screen now shows:
  Row 1: "DongLoRa" (bold)
  Row 2: "Board Name vX.Y.Z"
  Row 3: MAC address (XX:XX:XX:XX:XX:XX)
  Row 4: "Waiting for host..."

Each board reads its MAC at init:
- ESP32-S3: efuse MAC via esp_hal::efuse::Efuse::mac_address()
- nRF52840: FICR device address registers

BoardInfo struct bundles name/version/mac to keep dashboard() under
clippy's argument limit.

src/board/heltec_v3.rs

@@ -41,6 +41,7 @@ pub struct UsbParts {
 
 pub struct DisplayParts {
     pub i2c: DisplayI2c,
+    pub mac: [u8; 6],
 }
 
 // ── Board init ───────────────────────────────────────────────────────
@@ -130,7 +131,8 @@ impl Board {
             .with_sda(p.GPIO17)
             .with_scl(p.GPIO18)
             .into_async();
-        let display = Some(DisplayParts { i2c });
+        let mac = esp_hal::efuse::Efuse::mac_address();
+        let display = Some(DisplayParts { i2c, mac });
 
         (radio, usb, display)
     }

src/board/heltec_v4.rs

@@ -41,6 +41,7 @@ pub struct UsbParts {
 
 pub struct DisplayParts {
     pub i2c: DisplayI2c,
+    pub mac: [u8; 6],
 }
 
 // ── Board init ───────────────────────────────────────────────────────
@@ -141,7 +142,8 @@ impl Board {
             .with_sda(p.GPIO17)
             .with_scl(p.GPIO18)
             .into_async();
-        let display = Some(DisplayParts { i2c });
+        let mac = esp_hal::efuse::Efuse::mac_address();
+        let display = Some(DisplayParts { i2c, mac });
 
         (radio, usb, display)
     }

src/board/rak_wisblock_4631.rs

@@ -49,6 +49,7 @@ pub struct UsbParts {
 
 pub struct DisplayParts {
     pub i2c: DisplayI2c,
+    pub mac: [u8; 6],
 }
 
 // ── Board init ───────────────────────────────────────────────────────
@@ -110,7 +111,23 @@ impl Board {
         // via SSD1306 init (fails gracefully if no display attached).
         let i2c_cfg = twim::Config::default();
         let i2c = Twim::new(p.TWISPI0, Irqs, p.P0_13, p.P0_14, i2c_cfg);
-        let display = Some(DisplayParts { i2c });
+        // Read device address from FICR (factory info registers).
+        // FICR is at fixed address 0x10000000 on nRF52840.
+        let mac = unsafe {
+            let ficr = 0x10000000 as *const u32;
+            // DEVICEADDR[0] at offset 0x0A4, DEVICEADDR[1] at offset 0x0A8
+            let addr0 = core::ptr::read_volatile(ficr.byte_add(0x0A4));
+            let addr1 = core::ptr::read_volatile(ficr.byte_add(0x0A8));
+            [
+                addr0 as u8,
+                (addr0 >> 8) as u8,
+                (addr0 >> 16) as u8,
+                (addr0 >> 24) as u8,
+                addr1 as u8,
+                (addr1 >> 8) as u8,
+            ]
+        };
+        let display = Some(DisplayParts { i2c, mac });
 
         (radio, usb, display)
     }

src/display/render.rs

@@ -26,6 +26,13 @@ const SPARK_H: i32 = 19; // y=45..63
 const RSSI_MIN: i16 = -120;
 const RSSI_MAX: i16 = 0;
 
+/// Static board identity info for display rendering.
+pub struct BoardInfo<'a> {
+    pub name: &'a str,
+    pub version: &'a str,
+    pub mac: &'a str,
+}
+
 /// Render the main status dashboard.
 pub fn dashboard(
     target: &mut impl DrawTarget<Color = BinaryColor>,
@@ -33,16 +40,14 @@ pub fn dashboard(
     rssi_history: &[i16; RSSI_HISTORY_LEN],
     tx_history: &[bool; RSSI_HISTORY_LEN],
     rssi_count: usize,
-    board_name: &str,
-    version: &str,
+    board: &BoardInfo,
 ) {
     let _ = target.clear(BinaryColor::Off);
     let style = MonoTextStyle::new(&FONT_6X10, BinaryColor::On);
+    let mut buf: String<32> = String::new();
 
     match status.config {
         Some(cfg) => {
-            // Row 0: state + frequency, TX power right-justified
-            let mut buf: String<32> = String::new();
             let freq_mhz = cfg.freq_hz / 1_000_000;
             let freq_khz = (cfg.freq_hz % 1_000_000) / 1_000;
             let _ = write!(buf, "     {}.{:03}MHz", freq_mhz, freq_khz);
@@ -145,24 +150,30 @@ pub fn dashboard(
             rssi_sparkline(target, rssi_history, tx_history, rssi_count);
         }
         None => {
+            // Row 1: "DongLoRa" in bold
             let title_style = MonoTextStyle::new(&FONT_9X15_BOLD, BinaryColor::On);
             Text::with_alignment(
                 "DongLoRa",
-                Point::new(W / 2, 24),
+                Point::new(W / 2, 15),
                 title_style,
                 Alignment::Center,
             )
             .draw(target)
             .ok();
-            Text::with_alignment(version, Point::new(W / 2, 38), style, Alignment::Center)
+            // Row 2: "Board Name v0.1.0"
+            buf.clear();
+            let _ = write!(buf, "{} v{}", board.name, board.version);
+            Text::with_alignment(&buf, Point::new(W / 2, 28), style, Alignment::Center)
                 .draw(target)
                 .ok();
-            Text::with_alignment(board_name, Point::new(W / 2, 50), style, Alignment::Center)
+            // Row 3: MAC address
+            Text::with_alignment(board.mac, Point::new(W / 2, 41), style, Alignment::Center)
                 .draw(target)
                 .ok();
+            // Row 4: status
             Text::with_alignment(
                 "Waiting for host...",
-                Point::new(W / 2, 63),
+                Point::new(W / 2, 54),
                 style,
                 Alignment::Center,
             )

src/display/task.rs

@@ -81,6 +81,23 @@ pub async fn display_task(
     status: &'static StatusWatch,
     display_commands: &'static DisplayCommandChannel,
 ) {
+    // Format MAC address as "XX:XX:XX:XX:XX:XX"
+    let mut mac_str: heapless::String<18> = heapless::String::new();
+    let m = parts.mac;
+    let _ = core::fmt::Write::write_fmt(
+        &mut mac_str,
+        format_args!(
+            "{:02X}:{:02X}:{:02X}:{:02X}:{:02X}:{:02X}",
+            m[0], m[1], m[2], m[3], m[4], m[5]
+        ),
+    );
+
+    let board_info = render::BoardInfo {
+        name: BOARD_NAME,
+        version: env!("CARGO_PKG_VERSION"),
+        mac: &mac_str,
+    };
+
     let interface = I2CDisplayInterface::new(parts.i2c);
     let mut display = Ssd1306Async::new(interface, DisplaySize128x64, DisplayRotation::Rotate0)
         .into_buffered_graphics_mode();
@@ -104,7 +121,7 @@ pub async fn display_task(
     };
 
     // Show splash/waiting screen (dashboard with no config renders it)
-    render_and_flush(&mut display, &state).await;
+    render_and_flush(&mut display, &state, &board_info).await;
 
     loop {
         match select3(
@@ -129,7 +146,7 @@ pub async fn display_task(
                 state.last_status = radio_status;
 
                 if state.display_on {
-                    render_and_flush(&mut display, &state).await;
+                    render_and_flush(&mut display, &state, &board_info).await;
                 }
             }
             Either3::Second(cmd) => match cmd {
@@ -145,18 +162,18 @@ pub async fn display_task(
                     if let Some(s) = receiver.try_get() {
                         state.last_status = s;
                     }
-                    render_and_flush(&mut display, &state).await;
+                    render_and_flush(&mut display, &state, &board_info).await;
                 }
                 DisplayCommand::Reset => {
                     state = DisplayState::new();
-                    render_and_flush(&mut display, &state).await;
+                    render_and_flush(&mut display, &state, &board_info).await;
                 }
             },
             Either3::Third(()) => {
                 // Timer tick: advance sparkline slot
                 state.advance_slot();
                 if state.display_on {
-                    render_and_flush(&mut display, &state).await;
+                    render_and_flush(&mut display, &state, &board_info).await;
                 }
             }
         }
@@ -171,7 +188,7 @@ type Display<I> = ssd1306::Ssd1306Async<
     ssd1306::mode::BufferedGraphicsModeAsync<DisplaySize128x64>,
 >;
 
-async fn render_and_flush<I>(display: &mut Display<I>, state: &DisplayState)
+async fn render_and_flush<I>(display: &mut Display<I>, state: &DisplayState, board: &render::BoardInfo<'_>)
 where
     I: display_interface::AsyncWriteOnlyDataCommand,
 {
@@ -181,8 +198,7 @@ where
         &state.rssi_history,
         &state.tx_history,
         state.rssi_count,
-        BOARD_NAME,
-        env!("CARGO_PKG_VERSION"),
+        board,
     );
     let _ = display.flush().await;
 }