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/maybe-async-cfg/tests/ui/04-unit-test-util.rs

@@ -0,0 +1,37 @@
+#[maybe_async_cfg::maybe(sync(feature = "is_sync"), async(not(feature = "is_sync")))]
+async fn async_fn() -> bool {
+    true
+}
+
+#[maybe_async_cfg::maybe(
+    idents(async_fn),
+    sync(feature = "is_sync", test),
+    async(all(not(feature="is_sync"), feature = "async_std"), async_std::test),
+    async(all(not(feature="is_sync"), feature = "tokio"), tokio::test)
+)]
+async fn test_async_fn() {
+    let res = async_fn().await;
+    assert_eq!(res, true);
+}
+
+#[maybe_async_cfg::maybe(sync(feature = "is_sync", test), async(not(feature = "is_sync"), async_std::test))]
+async fn test_async_fn2() {
+    let res = async_fn().await;
+    assert_eq!(res, true);
+}
+
+#[maybe_async_cfg::maybe(sync(feature = "is_sync", test), async(not(feature = "is_sync"), async_std::test))]
+async fn test_async_fn3() {
+    let res = async_fn().await;
+    assert_eq!(res, true);
+}
+
+#[maybe_async_cfg::maybe(sync(feature = "is_sync", test), async(not(feature = "is_sync"), async_std::test))]
+async fn test_async_fn4() {
+    let res = async_fn().await;
+    assert_eq!(res, true);
+}
+
+fn main() {
+
+}