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/once_cell/examples/test_synchronization.rs

@@ -0,0 +1,38 @@
+//! Test if the OnceCell properly synchronizes.
+//! Needs to be run in release mode.
+//!
+//! We create a `Vec` with `N_ROUNDS` of `OnceCell`s. All threads will walk the `Vec`, and race to
+//! be the first one to initialize a cell.
+//! Every thread adds the results of the cells it sees to an accumulator, which is compared at the
+//! end.
+//! All threads should end up with the same result.
+
+use once_cell::sync::OnceCell;
+
+const N_THREADS: usize = 32;
+const N_ROUNDS: usize = 1_000_000;
+
+static CELLS: OnceCell<Vec<OnceCell<usize>>> = OnceCell::new();
+static RESULT: OnceCell<usize> = OnceCell::new();
+
+fn main() {
+    let start = std::time::Instant::now();
+    CELLS.get_or_init(|| vec![OnceCell::new(); N_ROUNDS]);
+    let threads =
+        (0..N_THREADS).map(|i| std::thread::spawn(move || thread_main(i))).collect::<Vec<_>>();
+    for thread in threads {
+        thread.join().unwrap();
+    }
+    println!("{:?}", start.elapsed());
+    println!("No races detected");
+}
+
+fn thread_main(i: usize) {
+    let cells = CELLS.get().unwrap();
+    let mut accum = 0;
+    for cell in cells.iter() {
+        let &value = cell.get_or_init(|| i);
+        accum += value;
+    }
+    assert_eq!(RESULT.get_or_init(|| accum), &accum);
+}