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/darling/tests/custom_bound.rs

@@ -0,0 +1,25 @@
+#![allow(dead_code)]
+
+use std::ops::Add;
+
+use darling::{FromDeriveInput, FromMeta};
+
+#[derive(Debug, Clone, FromMeta)]
+#[darling(bound = "T: FromMeta + Add")]
+struct Wrapper<T>(pub T);
+
+impl<T: Add> Add for Wrapper<T> {
+    type Output = Wrapper<<T as Add>::Output>;
+    fn add(self, rhs: Self) -> Wrapper<<T as Add>::Output> {
+        Wrapper(self.0 + rhs.0)
+    }
+}
+
+#[derive(Debug, FromDeriveInput)]
+#[darling(attributes(hello), bound = "Wrapper<T>: Add, T: FromMeta")]
+struct Foo<T> {
+    lorem: Wrapper<T>,
+}
+
+#[test]
+fn expansion() {}