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/happy_path.rs

@@ -0,0 +1,69 @@
+use darling::{FromDeriveInput, FromMeta};
+use syn::parse_quote;
+
+#[derive(Default, FromMeta, PartialEq, Debug)]
+#[darling(default)]
+struct Lorem {
+    ipsum: bool,
+    dolor: Option<String>,
+}
+
+#[derive(FromDeriveInput, PartialEq, Debug)]
+#[darling(attributes(darling_demo))]
+struct Core {
+    ident: syn::Ident,
+    vis: syn::Visibility,
+    generics: syn::Generics,
+    lorem: Lorem,
+}
+
+#[derive(FromDeriveInput, PartialEq, Debug)]
+#[darling(attributes(darling_demo))]
+struct TraitCore {
+    ident: syn::Ident,
+    generics: syn::Generics,
+    lorem: Lorem,
+}
+
+#[test]
+fn simple() {
+    let di = parse_quote! {
+        #[derive(Foo)]
+        #[darling_demo(lorem(ipsum))]
+        pub struct Bar;
+    };
+
+    assert_eq!(
+        Core::from_derive_input(&di).unwrap(),
+        Core {
+            ident: parse_quote!(Bar),
+            vis: parse_quote!(pub),
+            generics: Default::default(),
+            lorem: Lorem {
+                ipsum: true,
+                dolor: None,
+            },
+        }
+    );
+}
+
+#[test]
+fn trait_type() {
+    let di = parse_quote! {
+        #[derive(Foo)]
+        #[darling_demo(lorem(dolor = "hello"))]
+        pub struct Bar;
+    };
+
+    assert_eq!(
+        TraitCore::from_derive_input(&di).unwrap(),
+        TraitCore {
+            ident: parse_quote!(Bar),
+            generics: Default::default(),
+            lorem: Lorem {
+                ipsum: false,
+                dolor: Some("hello".to_owned()),
+            }
+        }
+    );
+}