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-executor/tests/test.rs

@@ -0,0 +1,151 @@
+#![cfg_attr(feature = "nightly", feature(impl_trait_in_assoc_type))]
+
+use std::boxed::Box;
+use std::future::poll_fn;
+use std::sync::{Arc, Mutex};
+use std::task::Poll;
+
+use embassy_executor::raw::Executor;
+use embassy_executor::task;
+
+#[export_name = "__pender"]
+fn __pender(context: *mut ()) {
+    unsafe {
+        let trace = &*(context as *const Trace);
+        trace.push("pend");
+    }
+}
+
+#[derive(Clone)]
+struct Trace {
+    trace: Arc<Mutex<Vec<&'static str>>>,
+}
+
+impl Trace {
+    fn new() -> Self {
+        Self {
+            trace: Arc::new(Mutex::new(Vec::new())),
+        }
+    }
+    fn push(&self, value: &'static str) {
+        self.trace.lock().unwrap().push(value)
+    }
+
+    fn get(&self) -> Vec<&'static str> {
+        self.trace.lock().unwrap().clone()
+    }
+}
+
+fn setup() -> (&'static Executor, Trace) {
+    let trace = Trace::new();
+    let context = Box::leak(Box::new(trace.clone())) as *mut _ as *mut ();
+    let executor = &*Box::leak(Box::new(Executor::new(context)));
+    (executor, trace)
+}
+
+#[test]
+fn executor_noop() {
+    let (executor, trace) = setup();
+    unsafe { executor.poll() };
+    assert!(trace.get().is_empty())
+}
+
+#[test]
+fn executor_task() {
+    #[task]
+    async fn task1(trace: Trace) {
+        trace.push("poll task1")
+    }
+
+    let (executor, trace) = setup();
+    executor.spawner().spawn(task1(trace.clone())).unwrap();
+
+    unsafe { executor.poll() };
+    unsafe { executor.poll() };
+
+    assert_eq!(
+        trace.get(),
+        &[
+            "pend",       // spawning a task pends the executor
+            "poll task1", // poll only once.
+        ]
+    )
+}
+
+#[test]
+fn executor_task_self_wake() {
+    #[task]
+    async fn task1(trace: Trace) {
+        poll_fn(|cx| {
+            trace.push("poll task1");
+            cx.waker().wake_by_ref();
+            Poll::Pending
+        })
+        .await
+    }
+
+    let (executor, trace) = setup();
+    executor.spawner().spawn(task1(trace.clone())).unwrap();
+
+    unsafe { executor.poll() };
+    unsafe { executor.poll() };
+
+    assert_eq!(
+        trace.get(),
+        &[
+            "pend",       // spawning a task pends the executor
+            "poll task1", //
+            "pend",       // task self-wakes
+            "poll task1", //
+            "pend",       // task self-wakes
+        ]
+    )
+}
+
+#[test]
+fn executor_task_self_wake_twice() {
+    #[task]
+    async fn task1(trace: Trace) {
+        poll_fn(|cx| {
+            trace.push("poll task1");
+            cx.waker().wake_by_ref();
+            trace.push("poll task1 wake 2");
+            cx.waker().wake_by_ref();
+            Poll::Pending
+        })
+        .await
+    }
+
+    let (executor, trace) = setup();
+    executor.spawner().spawn(task1(trace.clone())).unwrap();
+
+    unsafe { executor.poll() };
+    unsafe { executor.poll() };
+
+    assert_eq!(
+        trace.get(),
+        &[
+            "pend",              // spawning a task pends the executor
+            "poll task1",        //
+            "pend",              // task self-wakes
+            "poll task1 wake 2", // task self-wakes again, shouldn't pend
+            "poll task1",        //
+            "pend",              // task self-wakes
+            "poll task1 wake 2", // task self-wakes again, shouldn't pend
+        ]
+    )
+}
+
+#[test]
+fn executor_task_cfg_args() {
+    // simulate cfg'ing away argument c
+    #[task]
+    async fn task1(a: u32, b: u32, #[cfg(any())] c: u32) {
+        let (_, _) = (a, b);
+    }
+
+    #[task]
+    async fn task2(a: u32, b: u32, #[cfg(all())] c: u32) {
+        let (_, _, _) = (a, b, c);
+    }
+}