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/src/lib.rs

@@ -0,0 +1,76 @@
+//! A generic display interface
+//!
+//! This crate contains an error type and traits to implement for bus interface drivers drivers to
+//! be consumed by display drivers. It abstracts over the different communication methods available
+//! to drive a display and allows a driver writer to focus on driving the display itself and only
+//! have to implement a single interface.
+
+#![no_std]
+#![allow(async_fn_in_trait)]
+
+pub mod prelude;
+
+/// A ubiquitous error type for all kinds of problems which could happen when communicating with a
+/// display
+#[derive(Clone, Debug)]
+#[cfg_attr(feature = "defmt-03", derive(defmt::Format))]
+#[non_exhaustive]
+pub enum DisplayError {
+    /// Invalid data format selected for interface selected
+    InvalidFormatError,
+    /// Unable to write to bus
+    BusWriteError,
+    /// Unable to assert or de-assert data/command switching signal
+    DCError,
+    /// Unable to assert chip select signal
+    CSError,
+    /// The requested DataFormat is not implemented by this display interface implementation
+    DataFormatNotImplemented,
+    /// Unable to assert or de-assert reset signal
+    RSError,
+    /// Attempted to write to a non-existing pixel outside the display's bounds
+    OutOfBoundsError,
+}
+
+/// DI specific data format wrapper around slices of various widths
+/// Display drivers need to implement non-trivial conversions (e.g. with padding)
+/// as the hardware requires.
+#[non_exhaustive]
+pub enum DataFormat<'a> {
+    /// Slice of unsigned bytes
+    U8(&'a [u8]),
+    /// Slice of unsigned 16bit values with the same endianness as the system, not recommended
+    U16(&'a [u16]),
+    /// Slice of unsigned 16bit values to be sent in big endian byte order
+    U16BE(&'a mut [u16]),
+    /// Slice of unsigned 16bit values to be sent in little endian byte order
+    U16LE(&'a mut [u16]),
+    /// Iterator over unsigned bytes
+    U8Iter(&'a mut dyn Iterator<Item = u8>),
+    /// Iterator over unsigned 16bit values to be sent in big endian byte order
+    U16BEIter(&'a mut dyn Iterator<Item = u16>),
+    /// Iterator over unsigned 16bit values to be sent in little endian byte order
+    U16LEIter(&'a mut dyn Iterator<Item = u16>),
+}
+
+/// This trait implements a write-only interface for a display which has separate data and command
+/// modes. It is the responsibility of implementations to activate the correct mode in their
+/// implementation when corresponding method is called.
+pub trait WriteOnlyDataCommand {
+    /// Send a batch of commands to display
+    fn send_commands(&mut self, cmd: DataFormat<'_>) -> Result<(), DisplayError>;
+
+    /// Send pixel data to display
+    fn send_data(&mut self, buf: DataFormat<'_>) -> Result<(), DisplayError>;
+}
+
+/// This trait implements a write-only interface for a display which has separate data and command
+/// modes. It is the responsibility of implementations to activate the correct mode in their
+/// implementation when corresponding method is called.
+pub trait AsyncWriteOnlyDataCommand {
+    /// Send a batch of commands to display
+    async fn send_commands(&mut self, cmd: DataFormat<'_>) -> Result<(), DisplayError>;
+
+    /// Send pixel data to display
+    async fn send_data(&mut self, buf: DataFormat<'_>) -> Result<(), DisplayError>;
+}

vendor/display-interface/src/prelude.rs

@@ -0,0 +1,3 @@
+pub use crate::AsyncWriteOnlyDataCommand as _display_interface_AsyncWriteOnlyDataCommand;
+pub use crate::DisplayError as _display_interface_DisplayError;
+pub use crate::WriteOnlyDataCommand as _display_interface_WriteOnlyDataCommand;