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/generics_with.rs

@@ -0,0 +1,77 @@
+use std::collections::BTreeSet;
+
+use darling::{Error, FromDeriveInput, Result};
+use syn::{parse_quote, Ident};
+
+fn check_ident(ident: &Ident) -> Result<String> {
+    let s = ident.to_string();
+    if s.len() < 2 {
+        Err(Error::custom("generics must be at least 2 characters").with_span(ident))
+    } else {
+        Ok(s)
+    }
+}
+
+fn long_generic_names(generics: &syn::Generics) -> Result<BTreeSet<String>> {
+    let mut errors = Error::accumulator();
+    let valid = generics
+        .type_params()
+        .filter_map(|c| errors.handle(check_ident(&c.ident)))
+        .collect();
+    errors.finish_with(valid)
+}
+
+#[derive(Debug, FromDeriveInput)]
+#[darling(attributes(a))]
+struct Receiver {
+    #[darling(with = long_generic_names)]
+    generics: BTreeSet<String>,
+    surname: String,
+}
+
+#[test]
+fn succeeds_on_no_generics() {
+    let di = Receiver::from_derive_input(&parse_quote! {
+        #[a(surname = "Smith")]
+        struct Demo;
+    })
+    .unwrap();
+
+    assert!(di.generics.is_empty());
+}
+
+#[test]
+fn succeeds_on_valid_generics() {
+    let di = Receiver::from_derive_input(&parse_quote! {
+        #[a(surname = "Smith")]
+        struct Demo<Greeting> {
+            hello: Greeting,
+            world: String,
+        }
+    })
+    .unwrap();
+
+    assert_eq!(di.generics.len(), 1);
+    assert!(di.generics.contains("Greeting"));
+    assert_eq!(di.surname, "Smith");
+}
+
+#[test]
+fn rejects_invalid_input() {
+    let err = Receiver::from_derive_input(&parse_quote! {
+        struct Demo<G, S> {
+            hello: G,
+            world: S,
+        }
+    })
+    .unwrap_err();
+
+    assert_eq!(
+        err.len(),
+        // 2 errors from short type param names
+        2 +
+        // error for missing field `surname`
+        1,
+        "errors should have accumulated, and body checking should have occurred"
+    );
+}