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/tests.rs

@@ -0,0 +1,34 @@
+use crate as defmt;
+
+#[test]
+fn log_levels() {
+    // just make sure they build OK for now
+    defmt::trace!("test trace");
+    defmt::debug!("test debug");
+    defmt::info!("test info");
+    defmt::warn!("test warn");
+    defmt::error!("test error");
+}
+
+#[test]
+fn str() {
+    defmt::info!("Hello, {=str}", "world");
+
+    let world = defmt::intern!("world");
+    defmt::info!("Hello, {=istr}", world);
+}
+
+#[test]
+fn trailing_comma() {
+    defmt::trace!("test trace",);
+    defmt::debug!("test debug",);
+    defmt::info!("test info",);
+    defmt::warn!("test warn",);
+    defmt::error!("test error",);
+
+    defmt::trace!("test trace {=?}", 0,);
+    defmt::debug!("test debug {=?}", 0,);
+    defmt::info!("test info {=?}", 0,);
+    defmt::warn!("test warn {=?}", 0,);
+    defmt::error!("test error {=?}", 0,);
+}