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/override_callable.rs

@@ -0,0 +1,50 @@
+//! This is a regression test for issue [#371](https://github.com/TedDriggs/darling/issues/371).
+
+use darling::util::{Callable, Override};
+use darling::FromDeriveInput;
+
+#[derive(FromDeriveInput)]
+#[darling(attributes(test))]
+struct Test1 {
+    #[allow(dead_code)]
+    func: Callable,
+}
+
+#[derive(FromDeriveInput)]
+#[darling(attributes(test))]
+struct Test2 {
+    func: Override<Callable>,
+}
+
+#[test]
+fn test_explicit_closure() {
+    let input = syn::parse_quote! {
+        #[test(func = || 1 + 1)]
+        struct Foo;
+    };
+    assert!(Test1::from_derive_input(&input).is_ok());
+    assert!(Test2::from_derive_input(&input).is_ok());
+}
+
+#[test]
+fn test_explicit_path() {
+    let input = syn::parse_quote! {
+        #[test(func = foo::bar)]
+        struct Foo;
+    };
+    assert!(Test1::from_derive_input(&input).is_ok());
+    assert!(Test2::from_derive_input(&input).is_ok());
+}
+
+#[test]
+fn test_inherit() {
+    let input = syn::parse_quote! {
+        #[test(func)]
+        struct Foo;
+    };
+    assert!(Test1::from_derive_input(&input).is_err());
+    assert!(matches!(
+        Test2::from_derive_input(&input).unwrap().func,
+        Override::Inherit
+    ));
+}