feat: add display UI with splash screen, dashboard, and RSSI sparkline

Implement full SSD1306 OLED display support:
- Splash screen on boot (1.5s) with project name, version, board
- Dashboard showing radio state, config, counters, RSSI/SNR
- RSSI history sparkline (64-sample ring buffer, 2px-wide bars)
- DisplayOn/DisplayOff host commands routed via dedicated channel
- Pure render functions against generic DrawTarget (no hardware coupling)
- Display on/off control bypasses radio task entirely

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/display-interface-i2c/src/asynch.rs

@@ -0,0 +1,85 @@
+use display_interface::{AsyncWriteOnlyDataCommand, DataFormat, DisplayError};
+
+use crate::I2CInterface;
+
+impl<I2C> AsyncWriteOnlyDataCommand for I2CInterface<I2C>
+where
+    I2C: embedded_hal_async::i2c::I2c,
+{
+    async fn send_commands(&mut self, cmds: DataFormat<'_>) -> Result<(), DisplayError> {
+        // Copy over given commands to new aray to prefix with command identifier
+        match cmds {
+            DataFormat::U8(slice) => {
+                let mut writebuf: [u8; 8] = [0; 8];
+                writebuf[1..=slice.len()].copy_from_slice(&slice[0..slice.len()]);
+
+                self.i2c
+                    .write(self.addr, &writebuf[..=slice.len()])
+                    .await
+                    .map_err(|_| DisplayError::BusWriteError)
+            }
+            _ => Err(DisplayError::DataFormatNotImplemented),
+        }
+    }
+
+    async fn send_data(&mut self, buf: DataFormat<'_>) -> Result<(), DisplayError> {
+        match buf {
+            DataFormat::U8(slice) => {
+                // No-op if the data buffer is empty
+                if slice.is_empty() {
+                    return Ok(());
+                }
+
+                let mut writebuf = [0; 17];
+
+                // Data mode
+                writebuf[0] = self.data_byte;
+
+                for chunk in slice.chunks(16) {
+                    let chunk_len = chunk.len();
+
+                    // Copy over all data from buffer, leaving the data command byte intact
+                    writebuf[1..=chunk_len].copy_from_slice(chunk);
+
+                    self.i2c
+                        .write(self.addr, &writebuf[0..=chunk_len])
+                        .await
+                        .map_err(|_| DisplayError::BusWriteError)?;
+                }
+
+                Ok(())
+            }
+            DataFormat::U8Iter(iter) => {
+                let mut writebuf = [0; 17];
+                let mut i = 1;
+                let len = writebuf.len();
+
+                // Data mode
+                writebuf[0] = self.data_byte;
+
+                for byte in iter.into_iter() {
+                    writebuf[i] = byte;
+                    i += 1;
+
+                    if i == len {
+                        self.i2c
+                            .write(self.addr, &writebuf[0..=len])
+                            .await
+                            .map_err(|_| DisplayError::BusWriteError)?;
+                        i = 1;
+                    }
+                }
+
+                if i > 1 {
+                    self.i2c
+                        .write(self.addr, &writebuf[0..=i])
+                        .await
+                        .map_err(|_| DisplayError::BusWriteError)?;
+                }
+
+                Ok(())
+            }
+            _ => Err(DisplayError::DataFormatNotImplemented),
+        }
+    }
+}