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/futures-util/src/sink/flush.rs

@@ -0,0 +1,36 @@
+use core::marker::PhantomData;
+use core::pin::Pin;
+use futures_core::future::Future;
+use futures_core::task::{Context, Poll};
+use futures_sink::Sink;
+
+/// Future for the [`flush`](super::SinkExt::flush) method.
+#[derive(Debug)]
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+pub struct Flush<'a, Si: ?Sized, Item> {
+    sink: &'a mut Si,
+    _phantom: PhantomData<fn(Item)>,
+}
+
+// Pin is never projected to a field.
+impl<Si: Unpin + ?Sized, Item> Unpin for Flush<'_, Si, Item> {}
+
+/// A future that completes when the sink has finished processing all
+/// pending requests.
+///
+/// The sink itself is returned after flushing is complete; this adapter is
+/// intended to be used when you want to stop sending to the sink until
+/// all current requests are processed.
+impl<'a, Si: Sink<Item> + Unpin + ?Sized, Item> Flush<'a, Si, Item> {
+    pub(super) fn new(sink: &'a mut Si) -> Self {
+        Self { sink, _phantom: PhantomData }
+    }
+}
+
+impl<Si: Sink<Item> + Unpin + ?Sized, Item> Future for Flush<'_, Si, Item> {
+    type Output = Result<(), Si::Error>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        Pin::new(&mut self.sink).poll_flush(cx)
+    }
+}