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_path.rs

@@ -0,0 +1,54 @@
+#![cfg(feature = "std")]
+
+use core::fmt::Display;
+use ref_cast::RefCast;
+use std::path::{Path, PathBuf};
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error("failed to read '{file}'")]
+struct StructPathBuf {
+    file: PathBuf,
+}
+
+#[derive(Error, Debug, RefCast)]
+#[repr(C)]
+#[error("failed to read '{file}'")]
+struct StructPath {
+    file: Path,
+}
+
+#[derive(Error, Debug)]
+enum EnumPathBuf {
+    #[error("failed to read '{0}'")]
+    Read(PathBuf),
+}
+
+#[derive(Error, Debug)]
+#[error("{tail}")]
+pub struct UnsizedError {
+    pub head: i32,
+    pub tail: str,
+}
+
+#[derive(Error, Debug)]
+pub enum BothError {
+    #[error("display:{0} debug:{0:?}")]
+    DisplayDebug(PathBuf),
+    #[error("debug:{0:?} display:{0}")]
+    DebugDisplay(PathBuf),
+}
+
+fn assert<T: Display>(expected: &str, value: T) {
+    assert_eq!(expected, value.to_string());
+}
+
+#[test]
+fn test_display() {
+    let path = Path::new("/thiserror");
+    let file = path.to_owned();
+    assert("failed to read '/thiserror'", StructPathBuf { file });
+    let file = path.to_owned();
+    assert("failed to read '/thiserror'", EnumPathBuf::Read(file));
+    assert("failed to read '/thiserror'", StructPath::ref_cast(path));
+}