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/suggestions.rs

@@ -0,0 +1,44 @@
+#![allow(dead_code)]
+#![cfg(feature = "suggestions")]
+
+use darling::{FromDeriveInput, FromMeta};
+use syn::parse_quote;
+
+#[derive(Debug, FromDeriveInput)]
+#[darling(attributes(suggest))]
+struct Lorem {
+    ipsum: String,
+    dolor: Dolor,
+    // This field is included to make sure that skipped fields aren't suggested.
+    #[darling(skip)]
+    amet: bool,
+}
+
+#[derive(Debug, FromMeta)]
+struct Dolor {
+    sit: bool,
+}
+
+#[test]
+fn suggest_dolor() {
+    let input: syn::DeriveInput = parse_quote! {
+        #[suggest(ipsum = "Hello", dolorr(sit))]
+        pub struct Foo;
+    };
+
+    let result = Lorem::from_derive_input(&input).unwrap_err();
+    assert_eq!(2, result.len());
+    assert!(format!("{}", result).contains("Did you mean"));
+}
+
+#[test]
+fn dont_suggest_skipped_field() {
+    let input: syn::DeriveInput = parse_quote! {
+        #[suggest(ipsum = "Hello", dolor(sit), amt)]
+        pub struct Foo;
+    };
+
+    let result = Lorem::from_derive_input(&input).unwrap_err();
+    assert_eq!(1, result.len());
+    assert!(!format!("{}", result).contains("amet"));
+}