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/defmt/src/export/traits.rs

@@ -0,0 +1,82 @@
+#[allow(unused_imports)]
+use crate as defmt;
+use crate::{Format, Formatter, Str};
+
+pub trait Truncate<U> {
+    fn truncate(self) -> U;
+}
+
+macro_rules! impl_truncate {
+    ($($from:ty => $into:ty),*) => {
+        $(impl Truncate<$into> for $from {
+            fn truncate(self) -> $into {
+                self as $into
+            }
+        })*
+    };
+}
+
+// We implement `Truncate<X> for X` so that the macro can unconditionally use it,
+// even if no truncation is performed.
+impl_truncate!(
+    u8   => u8,
+    u16  => u8,
+    u32  => u8,
+    u64  => u8,
+    u128 => u8,
+    u16  => u16,
+    u32  => u16,
+    u64  => u16,
+    u128 => u16,
+    u32  => u32,
+    u64  => u32,
+    u128 => u32,
+    u64  => u64,
+    u128 => u64,
+    u128 => u128
+);
+
+#[derive(Debug, Copy, Clone, Eq, PartialEq)]
+pub struct NoneError;
+
+impl Format for NoneError {
+    fn format(&self, _fmt: Formatter) {
+        unreachable!();
+    }
+
+    fn _format_tag() -> Str {
+        defmt_macros::internp!("Unwrap of a None option value")
+    }
+
+    fn _format_data(&self) {}
+}
+
+/// Transform `self` into a `Result`
+///
+/// # Call sites
+/// * [`defmt::unwrap!`]
+pub trait IntoResult {
+    type Ok;
+    type Error;
+    fn into_result(self) -> Result<Self::Ok, Self::Error>;
+}
+
+impl<T> IntoResult for Option<T> {
+    type Ok = T;
+    type Error = NoneError;
+
+    #[inline]
+    fn into_result(self) -> Result<T, NoneError> {
+        self.ok_or(NoneError)
+    }
+}
+
+impl<T, E> IntoResult for Result<T, E> {
+    type Ok = T;
+    type Error = E;
+
+    #[inline]
+    fn into_result(self) -> Self {
+        self
+    }
+}