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-futures/src/yield_now.rs

@@ -0,0 +1,49 @@
+use core::future::Future;
+use core::pin::Pin;
+use core::task::{Context, Poll};
+
+/// Yield from the current task once, allowing other tasks to run.
+///
+/// This can be used to easily and quickly implement simple async primitives
+/// without using wakers. The following snippet will wait for a condition to
+/// hold, while still allowing other tasks to run concurrently (not monopolizing
+/// the executor thread).
+///
+/// ```rust
+/// # use embassy_futures::{block_on, yield_now};
+/// # async fn test_fn() {
+/// # let mut iter_count: u32 = 0;
+/// # let mut some_condition = || { iter_count += 1; iter_count > 10 };
+/// while !some_condition() {
+///     yield_now().await;
+/// }
+/// # }
+/// # block_on(test_fn());
+/// ```
+///
+/// The downside is this will spin in a busy loop, using 100% of the CPU, while
+/// using wakers correctly would allow the CPU to sleep while waiting.
+///
+/// The internal implementation is: on first poll the future wakes itself and
+/// returns `Poll::Pending`. On second poll, it returns `Poll::Ready`.
+pub fn yield_now() -> impl Future<Output = ()> {
+    YieldNowFuture { yielded: false }
+}
+
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+struct YieldNowFuture {
+    yielded: bool,
+}
+
+impl Future for YieldNowFuture {
+    type Output = ();
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        if self.yielded {
+            Poll::Ready(())
+        } else {
+            self.yielded = true;
+            cx.waker().wake_by_ref();
+            Poll::Pending
+        }
+    }
+}