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/data_with.rs

@@ -0,0 +1,49 @@
+use std::collections::BTreeSet;
+
+use darling::{Error, FromDeriveInput, Result};
+use syn::parse_quote;
+
+fn field_names(data: &syn::Data) -> Result<BTreeSet<String>> {
+    let fields = match data {
+        syn::Data::Struct(data) => data.fields.iter(),
+        syn::Data::Enum(_) => return Err(Error::custom("Expected struct or union")),
+        syn::Data::Union(data) => data.fields.named.iter(),
+    };
+
+    Ok(fields
+        .filter_map(|f| f.ident.clone())
+        .map(|i| i.to_string())
+        .collect())
+}
+
+#[derive(FromDeriveInput)]
+#[darling(attributes(a), forward_attrs)]
+struct Receiver {
+    #[darling(with = field_names)]
+    data: BTreeSet<String>,
+}
+
+#[test]
+fn succeeds_on_no_fields() {
+    let di = Receiver::from_derive_input(&parse_quote! {
+        struct Demo;
+    })
+    .unwrap();
+
+    assert!(di.data.is_empty());
+}
+
+#[test]
+fn succeeds_on_valid_input() {
+    let di = Receiver::from_derive_input(&parse_quote! {
+        struct Demo {
+            hello: String,
+            world: String,
+        }
+    })
+    .unwrap();
+
+    assert_eq!(di.data.len(), 2);
+    assert!(di.data.contains("hello"));
+    assert!(di.data.contains("world"));
+}