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/waitqueue/multi_waker.rs

@@ -0,0 +1,58 @@
+use core::task::Waker;
+
+use heapless::Vec;
+
+/// Utility struct to register and wake multiple wakers.
+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 the function returns it in the error
+    pub fn register<'a>(&mut self, w: &'a 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();
+        }
+    }
+}