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/defmt/src/formatter.rs

@@ -0,0 +1,16 @@
+use core::marker::PhantomData;
+
+/// Handle to a defmt logger.
+#[derive(Copy, Clone)]
+pub struct Formatter<'a> {
+    pub(crate) _phantom: PhantomData<&'a ()>,
+}
+
+/// An interned string created via [`intern!`].
+///
+/// [`intern!`]: macro.intern.html
+#[derive(Clone, Copy)]
+pub struct Str {
+    /// 16-bit address
+    pub(crate) address: u16,
+}