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/riscv/src/delay.rs

@@ -0,0 +1,30 @@
+//! Delay devices and providers
+use crate::register::mcycle;
+use embedded_hal::delay::DelayNs;
+
+/// Machine mode cycle counter (`mcycle`) as a delay provider
+#[derive(Copy, Clone)]
+#[repr(transparent)]
+pub struct McycleDelay {
+    /// The clock speed of the core, in Hertz
+    ticks_second: u32,
+}
+
+impl McycleDelay {
+    /// Constructs the delay provider.
+    /// `ticks_second` should be the clock speed of the core, in Hertz
+    #[inline]
+    pub const fn new(ticks_second: u32) -> Self {
+        Self { ticks_second }
+    }
+}
+
+impl DelayNs for McycleDelay {
+    #[inline]
+    fn delay_ns(&mut self, ns: u32) {
+        let t0 = mcycle::read64();
+        let ns_64: u64 = ns.into();
+        let clock = (ns_64 * (self.ticks_second as u64)) / 1_000_000_000u64;
+        while mcycle::read64().wrapping_sub(t0) <= clock {}
+    }
+}