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/tests/basic_usage.rs

@@ -0,0 +1,15 @@
+fn main() {
+    defmt::info!("hello");
+}
+
+#[defmt::global_logger]
+struct Logger;
+
+unsafe impl defmt::Logger for Logger {
+    fn acquire() {}
+    unsafe fn flush() {}
+    unsafe fn release() {}
+    unsafe fn write(_bytes: &[u8]) {}
+}
+
+defmt::timestamp!("{=u32}", 0);