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/bench.rs

@@ -0,0 +1,28 @@
+use std::mem::size_of;
+
+use once_cell::sync::OnceCell;
+
+const N_THREADS: usize = 32;
+const N_ROUNDS: usize = 100_000_000;
+
+static CELL: OnceCell<usize> = OnceCell::new();
+
+fn main() {
+    let start = std::time::Instant::now();
+    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!("size_of::<OnceCell<()>>()   = {:?}", size_of::<OnceCell<()>>());
+    println!("size_of::<OnceCell<bool>>() = {:?}", size_of::<OnceCell<bool>>());
+    println!("size_of::<OnceCell<u32>>()  = {:?}", size_of::<OnceCell<u32>>());
+}
+
+fn thread_main(i: usize) {
+    for _ in 0..N_ROUNDS {
+        let &value = CELL.get_or_init(|| i);
+        assert!(value < N_THREADS)
+    }
+}