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/computed_bound.rs

@@ -0,0 +1,42 @@
+use darling::{FromDeriveInput, FromMeta};
+
+fn parse<T: FromDeriveInput>(src: &str) -> T {
+    let ast = syn::parse_str(src).unwrap();
+    FromDeriveInput::from_derive_input(&ast).unwrap()
+}
+
+#[derive(FromMeta, PartialEq, Eq, Debug)]
+enum Volume {
+    Whisper,
+    Talk,
+    Shout,
+}
+
+#[derive(FromDeriveInput)]
+#[darling(attributes(speak))]
+struct SpeakingOptions<T: Default, U> {
+    max_volume: U,
+    #[darling(skip)]
+    #[allow(dead_code)]
+    additional_data: T,
+}
+
+#[derive(Default)]
+struct Phoneme {
+    #[allow(dead_code)]
+    first: String,
+}
+
+#[test]
+fn skipped_field() {
+    let parsed: SpeakingOptions<Phoneme, Volume> = parse(
+        r#"
+        #[derive(Speak)]
+        #[speak(max_volume = "shout")]
+        enum HtmlElement {
+            Div(String)
+        }
+    "#,
+    );
+    assert_eq!(parsed.max_volume, Volume::Shout);
+}