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/io/read_exact.rs

@@ -0,0 +1,42 @@
+use crate::io::AsyncRead;
+use futures_core::future::Future;
+use futures_core::ready;
+use futures_core::task::{Context, Poll};
+use std::io;
+use std::mem;
+use std::pin::Pin;
+
+/// Future for the [`read_exact`](super::AsyncReadExt::read_exact) method.
+#[derive(Debug)]
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+pub struct ReadExact<'a, R: ?Sized> {
+    reader: &'a mut R,
+    buf: &'a mut [u8],
+}
+
+impl<R: ?Sized + Unpin> Unpin for ReadExact<'_, R> {}
+
+impl<'a, R: AsyncRead + ?Sized + Unpin> ReadExact<'a, R> {
+    pub(super) fn new(reader: &'a mut R, buf: &'a mut [u8]) -> Self {
+        Self { reader, buf }
+    }
+}
+
+impl<R: AsyncRead + ?Sized + Unpin> Future for ReadExact<'_, R> {
+    type Output = io::Result<()>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let this = &mut *self;
+        while !this.buf.is_empty() {
+            let n = ready!(Pin::new(&mut this.reader).poll_read(cx, this.buf))?;
+            {
+                let (_, rest) = mem::take(&mut this.buf).split_at_mut(n);
+                this.buf = rest;
+            }
+            if n == 0 {
+                return Poll::Ready(Err(io::ErrorKind::UnexpectedEof.into()));
+            }
+        }
+        Poll::Ready(Ok(()))
+    }
+}