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/spin/examples/debug.rs

@@ -0,0 +1,21 @@
+extern crate spin;
+
+fn main() {
+    let mutex = spin::Mutex::new(42);
+    println!("{:?}", mutex);
+    {
+        let x = mutex.lock();
+        println!("{:?}, {:?}", mutex, *x);
+    }
+
+    let rwlock = spin::RwLock::new(42);
+    println!("{:?}", rwlock);
+    {
+        let x = rwlock.read();
+        println!("{:?}, {:?}", rwlock, *x);
+    }
+    {
+        let x = rwlock.write();
+        println!("{:?}, {:?}", rwlock, *x);
+    }
+}