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/cfg-if/tests/xcrate.rs

@@ -0,0 +1,16 @@
+#![allow(unexpected_cfgs)] // `foo` doesn't exist
+
+cfg_if::cfg_if! {
+    if #[cfg(foo)] {
+        fn works() -> bool { false }
+    } else if #[cfg(test)] {
+        fn works() -> bool { true }
+    } else {
+        fn works() -> bool { false }
+    }
+}
+
+#[test]
+fn smoke() {
+    assert!(works());
+}