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/src/waitqueue/atomic_waker.rs

@@ -0,0 +1,66 @@
+use core::cell::Cell;
+use core::task::Waker;
+
+use crate::blocking_mutex::raw::{CriticalSectionRawMutex, RawMutex};
+use crate::blocking_mutex::Mutex;
+
+/// Utility struct to register and wake a waker.
+/// If a waker is registered, registering another waker will replace the previous one without waking it.
+/// Intended to wake a task from an interrupt. Therefore, it is generally not expected,
+/// that multiple tasks register try to register a waker simultaneously.
+pub struct GenericAtomicWaker<M: RawMutex> {
+    waker: Mutex<M, Cell<Option<Waker>>>,
+}
+
+impl<M: RawMutex> GenericAtomicWaker<M> {
+    /// Create a new `AtomicWaker`.
+    pub const fn new(mutex: M) -> Self {
+        Self {
+            waker: Mutex::const_new(mutex, Cell::new(None)),
+        }
+    }
+
+    /// Register a waker. Overwrites the previous waker, if any.
+    pub fn register(&self, w: &Waker) {
+        self.waker.lock(|cell| {
+            cell.set(match cell.replace(None) {
+                Some(w2) if (w2.will_wake(w)) => Some(w2),
+                _ => Some(w.clone()),
+            })
+        })
+    }
+
+    /// Wake the registered waker, if any.
+    pub fn wake(&self) {
+        self.waker.lock(|cell| {
+            if let Some(w) = cell.replace(None) {
+                w.wake_by_ref();
+                cell.set(Some(w));
+            }
+        })
+    }
+}
+
+/// Utility struct to register and wake a waker.
+pub struct AtomicWaker {
+    waker: GenericAtomicWaker<CriticalSectionRawMutex>,
+}
+
+impl AtomicWaker {
+    /// Create a new `AtomicWaker`.
+    pub const fn new() -> Self {
+        Self {
+            waker: GenericAtomicWaker::new(CriticalSectionRawMutex::new()),
+        }
+    }
+
+    /// Register a waker. Overwrites the previous waker, if any.
+    pub fn register(&self, w: &Waker) {
+        self.waker.register(w);
+    }
+
+    /// Wake the registered waker, if any.
+    pub fn wake(&self) {
+        self.waker.wake();
+    }
+}

vendor/embassy-sync/src/waitqueue/atomic_waker_turbo.rs

@@ -0,0 +1,34 @@
+use core::ptr;
+use core::ptr::NonNull;
+use core::sync::atomic::{AtomicPtr, Ordering};
+use core::task::Waker;
+
+/// Utility struct to register and wake a waker.
+/// If a waker is registered, registering another waker will replace the previous one without waking it.
+/// The intended use case is to wake tasks from interrupts. Therefore, it is generally not expected,
+/// that multiple tasks register try to register a waker simultaneously.
+#[derive(Debug)]
+pub struct AtomicWaker {
+    waker: AtomicPtr<()>,
+}
+
+impl AtomicWaker {
+    /// Create a new `AtomicWaker`.
+    pub const fn new() -> Self {
+        Self {
+            waker: AtomicPtr::new(ptr::null_mut()),
+        }
+    }
+
+    /// Register a waker. Overwrites the previous waker, if any.
+    pub fn register(&self, w: &Waker) {
+        self.waker.store(w.as_turbo_ptr().as_ptr() as _, Ordering::Release);
+    }
+
+    /// Wake the registered waker, if any.
+    pub fn wake(&self) {
+        if let Some(ptr) = NonNull::new(self.waker.load(Ordering::Acquire)) {
+            unsafe { Waker::from_turbo_ptr(ptr) }.wake();
+        }
+    }
+}

vendor/embassy-sync/src/waitqueue/mod.rs

@@ -0,0 +1,11 @@
+//! Async low-level wait queues
+
+#[cfg_attr(feature = "turbowakers", path = "atomic_waker_turbo.rs")]
+mod atomic_waker;
+pub use atomic_waker::*;
+
+mod waker_registration;
+pub use waker_registration::*;
+
+mod multi_waker;
+pub use multi_waker::*;

vendor/embassy-sync/src/waitqueue/multi_waker.rs

@@ -0,0 +1,63 @@
+use core::task::Waker;
+
+use heapless::Vec;
+
+/// Utility struct to register and wake multiple wakers.
+/// Queue of wakers with a maximum length of `N`.
+/// Intended for waking multiple tasks.
+#[derive(Debug)]
+pub struct MultiWakerRegistration<const N: usize> {
+    wakers: Vec<Waker, N>,
+}
+
+impl<const N: usize> MultiWakerRegistration<N> {
+    /// Create a new empty instance
+    pub const fn new() -> Self {
+        Self { wakers: Vec::new() }
+    }
+
+    /// Register a waker.
+    ///
+    /// If the buffer is full, [wakes all the wakers](Self::wake), clears its buffer and registers the waker.
+    pub fn register(&mut self, w: &Waker) {
+        // If we already have some waker that wakes the same task as `w`, do nothing.
+        // This avoids cloning wakers, and avoids unnecessary mass-wakes.
+        for w2 in &self.wakers {
+            if w.will_wake(w2) {
+                return;
+            }
+        }
+
+        if self.wakers.is_full() {
+            // All waker slots were full. It's a bit inefficient, but we can wake everything.
+            // Any future that is still active will simply reregister.
+            // This won't happen a lot, so it's ok.
+            self.wake();
+        }
+
+        if self.wakers.push(w.clone()).is_err() {
+            // This can't happen unless N=0
+            // (Either `wakers` wasn't full, or it was in which case `wake()` empied it)
+            panic!("tried to push a waker to a zero-length MultiWakerRegistration")
+        }
+    }
+
+    /// Wake all registered wakers. This clears the buffer
+    pub fn wake(&mut self) {
+        // heapless::Vec has no `drain()`, do it unsafely ourselves...
+
+        // First set length to 0, without dropping the contents.
+        // This is necessary for soundness: if wake() panics and we're using panic=unwind.
+        // Setting len=0 upfront ensures other code can't observe the vec in an inconsistent state.
+        // (it'll leak wakers, but that's not UB)
+        let len = self.wakers.len();
+        unsafe { self.wakers.set_len(0) }
+
+        for i in 0..len {
+            // Move a waker out of the vec.
+            let waker = unsafe { self.wakers.as_mut_ptr().add(i).read() };
+            // Wake it by value, which consumes (drops) it.
+            waker.wake();
+        }
+    }
+}

vendor/embassy-sync/src/waitqueue/waker_registration.rs

@@ -0,0 +1,56 @@
+use core::mem;
+use core::task::Waker;
+
+/// Utility struct to register and wake a waker.
+/// If a waker is registered, registering another waker will replace the previous one.
+/// The previous waker will be woken in this case, giving it a chance to reregister itself.
+/// Although it is possible to wake multiple tasks this way,
+/// this will cause them to wake each other in a loop registering themselves.
+#[derive(Debug, Default)]
+pub struct WakerRegistration {
+    waker: Option<Waker>,
+}
+
+impl WakerRegistration {
+    /// Create a new `WakerRegistration`.
+    pub const fn new() -> Self {
+        Self { waker: None }
+    }
+
+    /// Register a waker. Overwrites the previous waker, if any.
+    pub fn register(&mut self, w: &Waker) {
+        match self.waker {
+            // Optimization: If both the old and new Wakers wake the same task, we can simply
+            // keep the old waker, skipping the clone. (In most executor implementations,
+            // cloning a waker is somewhat expensive, comparable to cloning an Arc).
+            Some(ref w2) if (w2.will_wake(w)) => {}
+            _ => {
+                // clone the new waker and store it
+                if let Some(old_waker) = mem::replace(&mut self.waker, Some(w.clone())) {
+                    // We had a waker registered for another task. Wake it, so the other task can
+                    // reregister itself if it's still interested.
+                    //
+                    // If two tasks are waiting on the same thing concurrently, this will cause them
+                    // to wake each other in a loop fighting over this WakerRegistration. This wastes
+                    // CPU but things will still work.
+                    //
+                    // If the user wants to have two tasks waiting on the same thing they should use
+                    // a more appropriate primitive that can store multiple wakers.
+                    old_waker.wake()
+                }
+            }
+        }
+    }
+
+    /// Wake the registered waker, if any.
+    pub fn wake(&mut self) {
+        if let Some(w) = self.waker.take() {
+            w.wake()
+        }
+    }
+
+    /// Returns true if a waker is currently registered
+    pub fn occupied(&self) -> bool {
+        self.waker.is_some()
+    }
+}