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-sync-0.5.0/src/zerocopy_channel.rs

@@ -0,0 +1,260 @@
+//! A zero-copy queue for sending values between asynchronous tasks.
+//!
+//! It can be used concurrently by multiple producers (senders) and multiple
+//! consumers (receivers), i.e. it is an  "MPMC channel".
+//!
+//! Receivers are competing for messages. So a message that is received by
+//! one receiver is not received by any other.
+//!
+//! This queue takes a Mutex type so that various
+//! targets can be attained. For example, a ThreadModeMutex can be used
+//! for single-core Cortex-M targets where messages are only passed
+//! between tasks running in thread mode. Similarly, a CriticalSectionMutex
+//! can also be used for single-core targets where messages are to be
+//! passed from exception mode e.g. out of an interrupt handler.
+//!
+//! This module provides a bounded channel that has a limit on the number of
+//! messages that it can store, and if this limit is reached, trying to send
+//! another message will result in an error being returned.
+
+use core::cell::RefCell;
+use core::future::poll_fn;
+use core::marker::PhantomData;
+use core::task::{Context, Poll};
+
+use crate::blocking_mutex::raw::RawMutex;
+use crate::blocking_mutex::Mutex;
+use crate::waitqueue::WakerRegistration;
+
+/// A bounded zero-copy channel for communicating between asynchronous tasks
+/// with backpressure.
+///
+/// The channel will buffer up to the provided number of messages.  Once the
+/// buffer is full, attempts to `send` new messages will wait until a message is
+/// received from the channel.
+///
+/// All data sent will become available in the same order as it was sent.
+///
+/// The channel requires a buffer of recyclable elements.  Writing to the channel is done through
+/// an `&mut T`.
+pub struct Channel<'a, M: RawMutex, T> {
+    buf: *mut T,
+    phantom: PhantomData<&'a mut T>,
+    state: Mutex<M, RefCell<State>>,
+}
+
+impl<'a, M: RawMutex, T> Channel<'a, M, T> {
+    /// Initialize a new [`Channel`].
+    ///
+    /// The provided buffer will be used and reused by the channel's logic, and thus dictates the
+    /// channel's capacity.
+    pub fn new(buf: &'a mut [T]) -> Self {
+        let len = buf.len();
+        assert!(len != 0);
+
+        Self {
+            buf: buf.as_mut_ptr(),
+            phantom: PhantomData,
+            state: Mutex::new(RefCell::new(State {
+                len,
+                front: 0,
+                back: 0,
+                full: false,
+                send_waker: WakerRegistration::new(),
+                receive_waker: WakerRegistration::new(),
+            })),
+        }
+    }
+
+    /// Creates a [`Sender`] and [`Receiver`] from an existing channel.
+    ///
+    /// Further Senders and Receivers can be created through [`Sender::borrow`] and
+    /// [`Receiver::borrow`] respectively.
+    pub fn split(&mut self) -> (Sender<'_, M, T>, Receiver<'_, M, T>) {
+        (Sender { channel: self }, Receiver { channel: self })
+    }
+}
+
+/// Send-only access to a [`Channel`].
+pub struct Sender<'a, M: RawMutex, T> {
+    channel: &'a Channel<'a, M, T>,
+}
+
+impl<'a, M: RawMutex, T> Sender<'a, M, T> {
+    /// Creates one further [`Sender`] over the same channel.
+    pub fn borrow(&mut self) -> Sender<'_, M, T> {
+        Sender { channel: self.channel }
+    }
+
+    /// Attempts to send a value over the channel.
+    pub fn try_send(&mut self) -> Option<&mut T> {
+        self.channel.state.lock(|s| {
+            let s = &mut *s.borrow_mut();
+            match s.push_index() {
+                Some(i) => Some(unsafe { &mut *self.channel.buf.add(i) }),
+                None => None,
+            }
+        })
+    }
+
+    /// Attempts to send a value over the channel.
+    pub fn poll_send(&mut self, cx: &mut Context) -> Poll<&mut T> {
+        self.channel.state.lock(|s| {
+            let s = &mut *s.borrow_mut();
+            match s.push_index() {
+                Some(i) => Poll::Ready(unsafe { &mut *self.channel.buf.add(i) }),
+                None => {
+                    s.receive_waker.register(cx.waker());
+                    Poll::Pending
+                }
+            }
+        })
+    }
+
+    /// Asynchronously send a value over the channel.
+    pub async fn send(&mut self) -> &mut T {
+        let i = poll_fn(|cx| {
+            self.channel.state.lock(|s| {
+                let s = &mut *s.borrow_mut();
+                match s.push_index() {
+                    Some(i) => Poll::Ready(i),
+                    None => {
+                        s.receive_waker.register(cx.waker());
+                        Poll::Pending
+                    }
+                }
+            })
+        })
+        .await;
+        unsafe { &mut *self.channel.buf.add(i) }
+    }
+
+    /// Notify the channel that the sending of the value has been finalized.
+    pub fn send_done(&mut self) {
+        self.channel.state.lock(|s| s.borrow_mut().push_done())
+    }
+}
+
+/// Receive-only access to a [`Channel`].
+pub struct Receiver<'a, M: RawMutex, T> {
+    channel: &'a Channel<'a, M, T>,
+}
+
+impl<'a, M: RawMutex, T> Receiver<'a, M, T> {
+    /// Creates one further [`Sender`] over the same channel.
+    pub fn borrow(&mut self) -> Receiver<'_, M, T> {
+        Receiver { channel: self.channel }
+    }
+
+    /// Attempts to receive a value over the channel.
+    pub fn try_receive(&mut self) -> Option<&mut T> {
+        self.channel.state.lock(|s| {
+            let s = &mut *s.borrow_mut();
+            match s.pop_index() {
+                Some(i) => Some(unsafe { &mut *self.channel.buf.add(i) }),
+                None => None,
+            }
+        })
+    }
+
+    /// Attempts to asynchronously receive a value over the channel.
+    pub fn poll_receive(&mut self, cx: &mut Context) -> Poll<&mut T> {
+        self.channel.state.lock(|s| {
+            let s = &mut *s.borrow_mut();
+            match s.pop_index() {
+                Some(i) => Poll::Ready(unsafe { &mut *self.channel.buf.add(i) }),
+                None => {
+                    s.send_waker.register(cx.waker());
+                    Poll::Pending
+                }
+            }
+        })
+    }
+
+    /// Asynchronously receive a value over the channel.
+    pub async fn receive(&mut self) -> &mut T {
+        let i = poll_fn(|cx| {
+            self.channel.state.lock(|s| {
+                let s = &mut *s.borrow_mut();
+                match s.pop_index() {
+                    Some(i) => Poll::Ready(i),
+                    None => {
+                        s.send_waker.register(cx.waker());
+                        Poll::Pending
+                    }
+                }
+            })
+        })
+        .await;
+        unsafe { &mut *self.channel.buf.add(i) }
+    }
+
+    /// Notify the channel that the receiving of the value has been finalized.
+    pub fn receive_done(&mut self) {
+        self.channel.state.lock(|s| s.borrow_mut().pop_done())
+    }
+}
+
+struct State {
+    len: usize,
+
+    /// Front index. Always 0..=(N-1)
+    front: usize,
+    /// Back index. Always 0..=(N-1).
+    back: usize,
+
+    /// Used to distinguish "empty" and "full" cases when `front == back`.
+    /// May only be `true` if `front == back`, always `false` otherwise.
+    full: bool,
+
+    send_waker: WakerRegistration,
+    receive_waker: WakerRegistration,
+}
+
+impl State {
+    fn increment(&self, i: usize) -> usize {
+        if i + 1 == self.len {
+            0
+        } else {
+            i + 1
+        }
+    }
+
+    fn is_full(&self) -> bool {
+        self.full
+    }
+
+    fn is_empty(&self) -> bool {
+        self.front == self.back && !self.full
+    }
+
+    fn push_index(&mut self) -> Option<usize> {
+        match self.is_full() {
+            true => None,
+            false => Some(self.back),
+        }
+    }
+
+    fn push_done(&mut self) {
+        assert!(!self.is_full());
+        self.back = self.increment(self.back);
+        if self.back == self.front {
+            self.full = true;
+        }
+        self.send_waker.wake();
+    }
+
+    fn pop_index(&mut self) -> Option<usize> {
+        match self.is_empty() {
+            true => None,
+            false => Some(self.front),
+        }
+    }
+
+    fn pop_done(&mut self) {
+        assert!(!self.is_empty());
+        self.front = self.increment(self.front);
+        self.full = false;
+        self.receive_waker.wake();
+    }
+}