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_error.rs

@@ -0,0 +1,56 @@
+#![allow(dead_code)]
+
+use core::fmt::{self, Display};
+use std::io;
+use thiserror::Error;
+
+macro_rules! unimplemented_display {
+    ($ty:ty) => {
+        impl Display for $ty {
+            fn fmt(&self, _formatter: &mut fmt::Formatter) -> fmt::Result {
+                unimplemented!()
+            }
+        }
+    };
+}
+
+#[derive(Error, Debug)]
+struct BracedError {
+    msg: String,
+    pos: usize,
+}
+
+#[derive(Error, Debug)]
+struct TupleError(String, usize);
+
+#[derive(Error, Debug)]
+struct UnitError;
+
+#[derive(Error, Debug)]
+struct WithSource {
+    #[source]
+    cause: io::Error,
+}
+
+#[derive(Error, Debug)]
+struct WithAnyhow {
+    #[source]
+    cause: anyhow::Error,
+}
+
+#[derive(Error, Debug)]
+enum EnumError {
+    Braced {
+        #[source]
+        cause: io::Error,
+    },
+    Tuple(#[source] io::Error),
+    Unit,
+}
+
+unimplemented_display!(BracedError);
+unimplemented_display!(TupleError);
+unimplemented_display!(UnitError);
+unimplemented_display!(WithSource);
+unimplemented_display!(WithAnyhow);
+unimplemented_display!(EnumError);