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/heapless/src/test_helpers.rs

@@ -0,0 +1,23 @@
+macro_rules! droppable {
+    () => {
+        static COUNT: core::sync::atomic::AtomicI32 = core::sync::atomic::AtomicI32::new(0);
+
+        #[derive(Eq, Ord, PartialEq, PartialOrd)]
+        struct Droppable(i32);
+        impl Droppable {
+            fn new() -> Self {
+                COUNT.fetch_add(1, core::sync::atomic::Ordering::Relaxed);
+                Droppable(Self::count())
+            }
+
+            fn count() -> i32 {
+                COUNT.load(core::sync::atomic::Ordering::Relaxed)
+            }
+        }
+        impl Drop for Droppable {
+            fn drop(&mut self) {
+                COUNT.fetch_sub(1, core::sync::atomic::Ordering::Relaxed);
+            }
+        }
+    };
+}