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/embassy-hal-internal/src/ratio.rs

@@ -0,0 +1,130 @@
+//! Types for dealing with rational numbers.
+use core::ops::{Add, Div, Mul};
+
+use num_traits::{CheckedAdd, CheckedDiv, CheckedMul};
+
+/// Represents the ratio between two numbers.
+#[derive(Copy, Clone, Debug)]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+pub struct Ratio<T> {
+    /// Numerator.
+    numer: T,
+    /// Denominator.
+    denom: T,
+}
+
+impl<T> Ratio<T> {
+    /// Creates a new `Ratio`.
+    #[inline(always)]
+    pub const fn new_raw(numer: T, denom: T) -> Ratio<T> {
+        Ratio { numer, denom }
+    }
+
+    /// Gets an immutable reference to the numerator.
+    #[inline(always)]
+    pub const fn numer(&self) -> &T {
+        &self.numer
+    }
+
+    /// Gets an immutable reference to the denominator.
+    #[inline(always)]
+    pub const fn denom(&self) -> &T {
+        &self.denom
+    }
+}
+
+impl<T: CheckedDiv> Ratio<T> {
+    /// Converts to an integer, rounding towards zero.
+    #[inline(always)]
+    pub fn to_integer(&self) -> T {
+        unwrap!(self.numer().checked_div(self.denom()))
+    }
+}
+
+impl<T: CheckedMul> Div<T> for Ratio<T> {
+    type Output = Self;
+
+    #[inline(always)]
+    fn div(mut self, rhs: T) -> Self::Output {
+        self.denom = unwrap!(self.denom().checked_mul(&rhs));
+        self
+    }
+}
+
+impl<T: CheckedMul> Mul<T> for Ratio<T> {
+    type Output = Self;
+
+    #[inline(always)]
+    fn mul(mut self, rhs: T) -> Self::Output {
+        self.numer = unwrap!(self.numer().checked_mul(&rhs));
+        self
+    }
+}
+
+impl<T: CheckedMul + CheckedAdd> Add<T> for Ratio<T> {
+    type Output = Self;
+
+    #[inline(always)]
+    fn add(mut self, rhs: T) -> Self::Output {
+        self.numer = unwrap!(unwrap!(self.denom().checked_mul(&rhs)).checked_add(self.numer()));
+        self
+    }
+}
+
+macro_rules! impl_from_for_float {
+    ($from:ident) => {
+        impl From<Ratio<$from>> for f32 {
+            #[inline(always)]
+            fn from(r: Ratio<$from>) -> Self {
+                (r.numer as f32) / (r.denom as f32)
+            }
+        }
+
+        impl From<Ratio<$from>> for f64 {
+            #[inline(always)]
+            fn from(r: Ratio<$from>) -> Self {
+                (r.numer as f64) / (r.denom as f64)
+            }
+        }
+    };
+}
+
+impl_from_for_float!(u8);
+impl_from_for_float!(u16);
+impl_from_for_float!(u32);
+impl_from_for_float!(u64);
+impl_from_for_float!(u128);
+impl_from_for_float!(i8);
+impl_from_for_float!(i16);
+impl_from_for_float!(i32);
+impl_from_for_float!(i64);
+impl_from_for_float!(i128);
+
+impl<T: core::fmt::Display> core::fmt::Display for Ratio<T> {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        core::write!(f, "{} / {}", self.numer(), self.denom())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::Ratio;
+
+    #[test]
+    fn basics() {
+        let mut r = Ratio::new_raw(1, 2) + 2;
+        assert_eq!(*r.numer(), 5);
+        assert_eq!(*r.denom(), 2);
+        assert_eq!(r.to_integer(), 2);
+
+        r = r * 2;
+        assert_eq!(*r.numer(), 10);
+        assert_eq!(*r.denom(), 2);
+        assert_eq!(r.to_integer(), 5);
+
+        r = r / 2;
+        assert_eq!(*r.numer(), 10);
+        assert_eq!(*r.denom(), 4);
+        assert_eq!(r.to_integer(), 2);
+    }
+}