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/anyhow/tests/test_repr.rs

@@ -0,0 +1,30 @@
+#![allow(clippy::extra_unused_type_parameters)]
+
+mod drop;
+
+use self::drop::{DetectDrop, Flag};
+use anyhow::Error;
+use std::mem;
+
+#[test]
+fn test_error_size() {
+    assert_eq!(mem::size_of::<Error>(), mem::size_of::<usize>());
+}
+
+#[test]
+fn test_null_pointer_optimization() {
+    assert_eq!(mem::size_of::<Result<(), Error>>(), mem::size_of::<usize>());
+}
+
+#[test]
+fn test_autotraits() {
+    fn assert<E: Unpin + Send + Sync + 'static>() {}
+    assert::<Error>();
+}
+
+#[test]
+fn test_drop() {
+    let has_dropped = Flag::new();
+    drop(Error::new(DetectDrop::new(&has_dropped)));
+    assert!(has_dropped.get());
+}