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/enums_default.rs

@@ -0,0 +1,44 @@
+use darling::{FromDeriveInput, FromMeta};
+use syn::parse_quote;
+
+#[derive(Debug, FromMeta, PartialEq, Eq)]
+enum Dolor {
+    Sit,
+    #[darling(word)]
+    Amet,
+}
+
+impl Default for Dolor {
+    fn default() -> Self {
+        Dolor::Sit
+    }
+}
+
+#[derive(FromDeriveInput)]
+#[darling(attributes(hello))]
+struct Receiver {
+    #[darling(default)]
+    example: Dolor,
+}
+
+#[test]
+fn missing_meta() {
+    let di = Receiver::from_derive_input(&parse_quote! {
+        #[hello]
+        struct Example;
+    })
+    .unwrap();
+
+    assert_eq!(Dolor::Sit, di.example);
+}
+
+#[test]
+fn empty_meta() {
+    let di = Receiver::from_derive_input(&parse_quote! {
+        #[hello(example)]
+        struct Example;
+    })
+    .unwrap();
+
+    assert_eq!(Dolor::Amet, di.example);
+}