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_chain.rs

@@ -0,0 +1,69 @@
+use anyhow::{anyhow, Chain, Error};
+
+fn error() -> Error {
+    anyhow!({ 0 }).context(1).context(2).context(3)
+}
+
+#[test]
+fn test_iter() {
+    let e = error();
+    let mut chain = e.chain();
+    assert_eq!("3", chain.next().unwrap().to_string());
+    assert_eq!("2", chain.next().unwrap().to_string());
+    assert_eq!("1", chain.next().unwrap().to_string());
+    assert_eq!("0", chain.next().unwrap().to_string());
+    assert!(chain.next().is_none());
+    assert!(chain.next_back().is_none());
+}
+
+#[test]
+fn test_rev() {
+    let e = error();
+    let mut chain = e.chain().rev();
+    assert_eq!("0", chain.next().unwrap().to_string());
+    assert_eq!("1", chain.next().unwrap().to_string());
+    assert_eq!("2", chain.next().unwrap().to_string());
+    assert_eq!("3", chain.next().unwrap().to_string());
+    assert!(chain.next().is_none());
+    assert!(chain.next_back().is_none());
+}
+
+#[test]
+fn test_len() {
+    let e = error();
+    let mut chain = e.chain();
+    assert_eq!(4, chain.len());
+    assert_eq!((4, Some(4)), chain.size_hint());
+    assert_eq!("3", chain.next().unwrap().to_string());
+    assert_eq!(3, chain.len());
+    assert_eq!((3, Some(3)), chain.size_hint());
+    assert_eq!("0", chain.next_back().unwrap().to_string());
+    assert_eq!(2, chain.len());
+    assert_eq!((2, Some(2)), chain.size_hint());
+    assert_eq!("2", chain.next().unwrap().to_string());
+    assert_eq!(1, chain.len());
+    assert_eq!((1, Some(1)), chain.size_hint());
+    assert_eq!("1", chain.next_back().unwrap().to_string());
+    assert_eq!(0, chain.len());
+    assert_eq!((0, Some(0)), chain.size_hint());
+    assert!(chain.next().is_none());
+}
+
+#[test]
+fn test_default() {
+    let mut c = Chain::default();
+    assert!(c.next().is_none());
+}
+
+#[test]
+#[allow(clippy::redundant_clone)]
+fn test_clone() {
+    let e = error();
+    let mut chain = e.chain().clone();
+    assert_eq!("3", chain.next().unwrap().to_string());
+    assert_eq!("2", chain.next().unwrap().to_string());
+    assert_eq!("1", chain.next().unwrap().to_string());
+    assert_eq!("0", chain.next().unwrap().to_string());
+    assert!(chain.next().is_none());
+    assert!(chain.next_back().is_none());
+}