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/fanout.rs

@@ -0,0 +1,111 @@
+use core::fmt::{Debug, Formatter, Result as FmtResult};
+use core::pin::Pin;
+use futures_core::task::{Context, Poll};
+use futures_sink::Sink;
+use pin_project_lite::pin_project;
+
+pin_project! {
+    /// Sink that clones incoming items and forwards them to two sinks at the same time.
+    ///
+    /// Backpressure from any downstream sink propagates up, which means that this sink
+    /// can only process items as fast as its _slowest_ downstream sink.
+    #[must_use = "sinks do nothing unless polled"]
+    pub struct Fanout<Si1, Si2> {
+        #[pin]
+        sink1: Si1,
+        #[pin]
+        sink2: Si2
+    }
+}
+
+impl<Si1, Si2> Fanout<Si1, Si2> {
+    pub(super) fn new(sink1: Si1, sink2: Si2) -> Self {
+        Self { sink1, sink2 }
+    }
+
+    /// Get a shared reference to the inner sinks.
+    pub fn get_ref(&self) -> (&Si1, &Si2) {
+        (&self.sink1, &self.sink2)
+    }
+
+    /// Get a mutable reference to the inner sinks.
+    pub fn get_mut(&mut self) -> (&mut Si1, &mut Si2) {
+        (&mut self.sink1, &mut self.sink2)
+    }
+
+    /// Get a pinned mutable reference to the inner sinks.
+    pub fn get_pin_mut(self: Pin<&mut Self>) -> (Pin<&mut Si1>, Pin<&mut Si2>) {
+        let this = self.project();
+        (this.sink1, this.sink2)
+    }
+
+    /// Consumes this combinator, returning the underlying sinks.
+    ///
+    /// Note that this may discard intermediate state of this combinator,
+    /// so care should be taken to avoid losing resources when this is called.
+    pub fn into_inner(self) -> (Si1, Si2) {
+        (self.sink1, self.sink2)
+    }
+}
+
+impl<Si1: Debug, Si2: Debug> Debug for Fanout<Si1, Si2> {
+    fn fmt(&self, f: &mut Formatter<'_>) -> FmtResult {
+        f.debug_struct("Fanout").field("sink1", &self.sink1).field("sink2", &self.sink2).finish()
+    }
+}
+
+impl<Si1, Si2, Item> Sink<Item> for Fanout<Si1, Si2>
+where
+    Si1: Sink<Item>,
+    Item: Clone,
+    Si2: Sink<Item, Error = Si1::Error>,
+{
+    type Error = Si1::Error;
+
+    fn poll_ready(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        let this = self.project();
+
+        let sink1_ready = this.sink1.poll_ready(cx)?.is_ready();
+        let sink2_ready = this.sink2.poll_ready(cx)?.is_ready();
+        let ready = sink1_ready && sink2_ready;
+        if ready {
+            Poll::Ready(Ok(()))
+        } else {
+            Poll::Pending
+        }
+    }
+
+    fn start_send(self: Pin<&mut Self>, item: Item) -> Result<(), Self::Error> {
+        let this = self.project();
+
+        this.sink1.start_send(item.clone())?;
+        this.sink2.start_send(item)?;
+        Ok(())
+    }
+
+    fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        let this = self.project();
+
+        let sink1_ready = this.sink1.poll_flush(cx)?.is_ready();
+        let sink2_ready = this.sink2.poll_flush(cx)?.is_ready();
+        let ready = sink1_ready && sink2_ready;
+        if ready {
+            Poll::Ready(Ok(()))
+        } else {
+            Poll::Pending
+        }
+    }
+
+    fn poll_close(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        let this = self.project();
+
+        let sink1_ready = this.sink1.poll_close(cx)?.is_ready();
+        let sink2_ready = this.sink2.poll_close(cx)?.is_ready();
+        let ready = sink1_ready && sink2_ready;
+        if ready {
+            Poll::Ready(Ok(()))
+        } else {
+            Poll::Pending
+        }
+    }
+}