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/window.rs

@@ -0,0 +1,104 @@
+use std::ops::{Bound, Range, RangeBounds};
+
+/// An owned window around an underlying buffer.
+///
+/// Normally slices work great for considering sub-portions of a buffer, but
+/// unfortunately a slice is a *borrowed* type in Rust which has an associated
+/// lifetime. When working with future and async I/O these lifetimes are not
+/// always appropriate, and are sometimes difficult to store in tasks. This
+/// type strives to fill this gap by providing an "owned slice" around an
+/// underlying buffer of bytes.
+///
+/// A `Window<T>` wraps an underlying buffer, `T`, and has configurable
+/// start/end indexes to alter the behavior of the `AsRef<[u8]>` implementation
+/// that this type carries.
+///
+/// This type can be particularly useful when working with the `write_all`
+/// combinator in this crate. Data can be sliced via `Window`, consumed by
+/// `write_all`, and then earned back once the write operation finishes through
+/// the `into_inner` method on this type.
+#[derive(Debug)]
+pub struct Window<T> {
+    inner: T,
+    range: Range<usize>,
+}
+
+impl<T: AsRef<[u8]>> Window<T> {
+    /// Creates a new window around the buffer `t` defaulting to the entire
+    /// slice.
+    ///
+    /// Further methods can be called on the returned `Window<T>` to alter the
+    /// window into the data provided.
+    pub fn new(t: T) -> Self {
+        Self { range: 0..t.as_ref().len(), inner: t }
+    }
+
+    /// Gets a shared reference to the underlying buffer inside of this
+    /// `Window`.
+    pub fn get_ref(&self) -> &T {
+        &self.inner
+    }
+
+    /// Gets a mutable reference to the underlying buffer inside of this
+    /// `Window`.
+    pub fn get_mut(&mut self) -> &mut T {
+        &mut self.inner
+    }
+
+    /// Consumes this `Window`, returning the underlying buffer.
+    pub fn into_inner(self) -> T {
+        self.inner
+    }
+
+    /// Returns the starting index of this window into the underlying buffer
+    /// `T`.
+    pub fn start(&self) -> usize {
+        self.range.start
+    }
+
+    /// Returns the end index of this window into the underlying buffer
+    /// `T`.
+    pub fn end(&self) -> usize {
+        self.range.end
+    }
+
+    /// Changes the range of this window to the range specified.
+    ///
+    /// # Panics
+    ///
+    /// This method will panic if `range` is out of bounds for the underlying
+    /// slice or if [`start_bound()`] of `range` comes after the [`end_bound()`].
+    ///
+    /// [`start_bound()`]: std::ops::RangeBounds::start_bound
+    /// [`end_bound()`]: std::ops::RangeBounds::end_bound
+    pub fn set<R: RangeBounds<usize>>(&mut self, range: R) {
+        let start = match range.start_bound() {
+            Bound::Included(n) => *n,
+            Bound::Excluded(n) => *n + 1,
+            Bound::Unbounded => 0,
+        };
+        let end = match range.end_bound() {
+            Bound::Included(n) => *n + 1,
+            Bound::Excluded(n) => *n,
+            Bound::Unbounded => self.inner.as_ref().len(),
+        };
+
+        assert!(end <= self.inner.as_ref().len());
+        assert!(start <= end);
+
+        self.range.start = start;
+        self.range.end = end;
+    }
+}
+
+impl<T: AsRef<[u8]>> AsRef<[u8]> for Window<T> {
+    fn as_ref(&self) -> &[u8] {
+        &self.inner.as_ref()[self.range.start..self.range.end]
+    }
+}
+
+impl<T: AsMut<[u8]>> AsMut<[u8]> for Window<T> {
+    fn as_mut(&mut self) -> &mut [u8] {
+        &mut self.inner.as_mut()[self.range.start..self.range.end]
+    }
+}