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/test_fail/02-unknown-path.rs

@@ -0,0 +1,21 @@
+#[maybe_async_cfg::maybe(
+    sync(feature = "is_sync"), 
+    unknown(not(feature = "is_sync")),
+)]
+async fn async_fn() -> bool {
+    true
+}
+
+#[maybe_async_cfg::maybe(
+    idents(async_fn(fn)),
+    sync(feature = "is_sync"), 
+    async(not(feature = "is_sync")),
+)]
+async fn test_async_fn() {
+    let res = async_fn().await;
+    assert_eq!(res, true);
+}
+
+fn main() {
+
+}