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/03-must-be-sync.rs

@@ -0,0 +1,57 @@
+#![allow(dead_code)]
+
+#[maybe_async_cfg::maybe(sync(feature = "is_sync"))]
+trait Trait {
+    fn sync_fn() {}
+
+    async fn declare_async(&self);
+
+    async fn async_fn(&self) {
+        self.declare_async().await
+    }
+}
+
+#[maybe_async_cfg::maybe(sync(feature = "is_sync"))]
+pub trait PubTrait {
+    fn sync_fn() {}
+
+    async fn declare_async(&self);
+
+    async fn async_fn(&self) {
+        self.declare_async().await
+    }
+}
+
+#[maybe_async_cfg::maybe(keep_self, sync(feature = "is_sync"))]
+async fn async_fn() {}
+
+#[maybe_async_cfg::maybe(keep_self, sync(feature = "is_sync"))]
+pub async fn pub_async_fn() {}
+
+struct Struct;
+
+#[maybe_async_cfg::maybe(keep_self, idents(Trait), sync(feature = "is_sync"))]
+impl Trait for Struct {
+    fn sync_fn() {}
+
+    async fn declare_async(&self) {}
+
+    async fn async_fn(&self) {
+        async { self.declare_async().await }.await
+    }
+}
+
+#[cfg(feature = "is_sync")]
+fn main() -> std::result::Result<(), ()> {
+    let s = Struct;
+    s.declare_async();
+    s.async_fn();
+    async_fn();
+    pub_async_fn();
+    Ok(())
+}
+
+
+#[cfg(not(feature = "is_sync"))]
+#[async_std::main]
+async fn main() {}