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/thiserror/tests/test_source.rs

@@ -0,0 +1,82 @@
+use std::error::Error as StdError;
+use std::io;
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error("implicit source")]
+pub struct ImplicitSource {
+    source: io::Error,
+}
+
+#[derive(Error, Debug)]
+#[error("explicit source")]
+pub struct ExplicitSource {
+    source: String,
+    #[source]
+    io: io::Error,
+}
+
+#[derive(Error, Debug)]
+#[error("boxed source")]
+pub struct BoxedSource {
+    #[source]
+    source: Box<dyn StdError + Send + 'static>,
+}
+
+#[test]
+fn test_implicit_source() {
+    let io = io::Error::new(io::ErrorKind::Other, "oh no!");
+    let error = ImplicitSource { source: io };
+    error.source().unwrap().downcast_ref::<io::Error>().unwrap();
+}
+
+#[test]
+fn test_explicit_source() {
+    let io = io::Error::new(io::ErrorKind::Other, "oh no!");
+    let error = ExplicitSource {
+        source: String::new(),
+        io,
+    };
+    error.source().unwrap().downcast_ref::<io::Error>().unwrap();
+}
+
+#[test]
+fn test_boxed_source() {
+    let source = Box::new(io::Error::new(io::ErrorKind::Other, "oh no!"));
+    let error = BoxedSource { source };
+    error.source().unwrap().downcast_ref::<io::Error>().unwrap();
+}
+
+macro_rules! error_from_macro {
+    ($($variants:tt)*) => {
+        #[derive(Error)]
+        #[derive(Debug)]
+        pub enum MacroSource {
+            $($variants)*
+        }
+    }
+}
+
+// Test that we generate impls with the proper hygiene
+#[rustfmt::skip]
+error_from_macro! {
+    #[error("Something")]
+    Variant(#[from] io::Error)
+}
+
+#[test]
+fn test_not_source() {
+    #[derive(Error, Debug)]
+    #[error("{source} ==> {destination}")]
+    pub struct NotSource {
+        r#source: char,
+        destination: char,
+    }
+
+    let error = NotSource {
+        source: 'S',
+        destination: 'D',
+    };
+    assert_eq!(error.to_string(), "S ==> D");
+    assert!(error.source().is_none());
+}