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/embedded-hal/src/delay.rs

@@ -0,0 +1,62 @@
+//! Delays.
+
+/// Nanoseconds per microsecond
+const NANOS_PER_MICRO: u32 = 1_000;
+/// Nanoseconds per millisecond
+const NANOS_PER_MILLI: u32 = 1_000_000;
+
+/// Delay with up to nanosecond precision.
+pub trait DelayNs {
+    /// Pauses execution for at minimum `ns` nanoseconds. Pause can be longer
+    /// if the implementation requires it due to precision/timing issues.
+    fn delay_ns(&mut self, ns: u32);
+
+    /// Pauses execution for at minimum `us` microseconds. Pause can be longer
+    /// if the implementation requires it due to precision/timing issues.
+    fn delay_us(&mut self, mut us: u32) {
+        const MAX_MICROS: u32 = u32::MAX / NANOS_PER_MICRO;
+
+        // Avoid potential overflow if micro -> nano conversion is too large
+        while us > MAX_MICROS {
+            us -= MAX_MICROS;
+            self.delay_ns(MAX_MICROS * NANOS_PER_MICRO);
+        }
+
+        self.delay_ns(us * NANOS_PER_MICRO);
+    }
+
+    /// Pauses execution for at minimum `ms` milliseconds. Pause can be longer
+    /// if the implementation requires it due to precision/timing issues.
+    #[inline]
+    fn delay_ms(&mut self, mut ms: u32) {
+        const MAX_MILLIS: u32 = u32::MAX / NANOS_PER_MILLI;
+
+        // Avoid potential overflow if milli -> nano conversion is too large
+        while ms > MAX_MILLIS {
+            ms -= MAX_MILLIS;
+            self.delay_ns(MAX_MILLIS * NANOS_PER_MILLI);
+        }
+
+        self.delay_ns(ms * NANOS_PER_MILLI);
+    }
+}
+
+impl<T> DelayNs for &mut T
+where
+    T: DelayNs + ?Sized,
+{
+    #[inline]
+    fn delay_ns(&mut self, ns: u32) {
+        T::delay_ns(self, ns);
+    }
+
+    #[inline]
+    fn delay_us(&mut self, us: u32) {
+        T::delay_us(self, us);
+    }
+
+    #[inline]
+    fn delay_ms(&mut self, ms: u32) {
+        T::delay_ms(self, ms);
+    }
+}