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/multiple.rs

@@ -0,0 +1,30 @@
+use darling::{FromDeriveInput, FromMeta};
+use syn::parse_quote;
+
+#[derive(FromDeriveInput)]
+#[darling(attributes(hello))]
+#[allow(dead_code)]
+struct Lorem {
+    ident: syn::Ident,
+    ipsum: Ipsum,
+}
+
+#[derive(FromMeta)]
+struct Ipsum {
+    #[darling(multiple)]
+    dolor: Vec<String>,
+}
+
+#[test]
+fn expand_many() {
+    let di = parse_quote! {
+        #[hello(ipsum(dolor = "Hello", dolor = "World"))]
+        pub struct Baz;
+    };
+
+    let lorem: Lorem = Lorem::from_derive_input(&di).unwrap();
+    assert_eq!(
+        lorem.ipsum.dolor,
+        vec!["Hello".to_string(), "World".to_string()]
+    );
+}