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/embedded-hal-0.2.7/src/spi.rs

@@ -0,0 +1,78 @@
+//! Serial Peripheral Interface
+
+use nb;
+
+/// Full duplex (master mode)
+///
+/// # Notes
+///
+/// - It's the task of the user of this interface to manage the slave select lines
+///
+/// - Due to how full duplex SPI works each `read` call must be preceded by a `send` call.
+///
+/// - Some SPIs can work with 8-bit *and* 16-bit words. You can overload this trait with different
+/// `Word` types to allow operation in both modes.
+pub trait FullDuplex<Word> {
+    /// An enumeration of SPI errors
+    type Error;
+
+    /// Reads the word stored in the shift register
+    ///
+    /// **NOTE** A word must be sent to the slave before attempting to call this
+    /// method.
+    fn read(&mut self) -> nb::Result<Word, Self::Error>;
+
+    /// Sends a word to the slave
+    fn send(&mut self, word: Word) -> nb::Result<(), Self::Error>;
+}
+
+/// Clock polarity
+#[derive(Clone, Copy, PartialEq, Eq)]
+pub enum Polarity {
+    /// Clock signal low when idle
+    IdleLow,
+    /// Clock signal high when idle
+    IdleHigh,
+}
+
+/// Clock phase
+#[derive(Clone, Copy, PartialEq, Eq)]
+pub enum Phase {
+    /// Data in "captured" on the first clock transition
+    CaptureOnFirstTransition,
+    /// Data in "captured" on the second clock transition
+    CaptureOnSecondTransition,
+}
+
+/// SPI mode
+#[derive(Clone, Copy, PartialEq, Eq)]
+pub struct Mode {
+    /// Clock polarity
+    pub polarity: Polarity,
+    /// Clock phase
+    pub phase: Phase,
+}
+
+/// Helper for CPOL = 0, CPHA = 0
+pub const MODE_0: Mode = Mode {
+    polarity: Polarity::IdleLow,
+    phase: Phase::CaptureOnFirstTransition,
+};
+
+/// Helper for CPOL = 0, CPHA = 1
+pub const MODE_1: Mode = Mode {
+    polarity: Polarity::IdleLow,
+    phase: Phase::CaptureOnSecondTransition,
+};
+
+/// Helper for CPOL = 1, CPHA = 0
+pub const MODE_2: Mode = Mode {
+    polarity: Polarity::IdleHigh,
+    phase: Phase::CaptureOnFirstTransition,
+};
+
+/// Helper for CPOL = 1, CPHA = 1
+pub const MODE_3: Mode = Mode {
+    polarity: Polarity::IdleHigh,
+    phase: Phase::CaptureOnSecondTransition,
+};