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/attrs_with.rs

@@ -0,0 +1,80 @@
+use std::collections::BTreeSet;
+
+use darling::{util, Error, FromDeriveInput, Result};
+use syn::{parse_quote, Attribute};
+
+fn unique_idents(attrs: Vec<Attribute>) -> Result<BTreeSet<String>> {
+    let mut errors = Error::accumulator();
+    let idents = attrs
+        .into_iter()
+        .filter_map(|attr| {
+            let path = attr.path();
+            errors.handle(
+                path.get_ident()
+                    .map(std::string::ToString::to_string)
+                    .ok_or_else(|| {
+                        Error::custom(format!("`{}` is not an ident", util::path_to_string(path)))
+                            .with_span(path)
+                    }),
+            )
+        })
+        .collect();
+
+    errors.finish_with(idents)
+}
+
+#[derive(FromDeriveInput)]
+#[darling(attributes(a), forward_attrs)]
+struct Receiver {
+    #[darling(with = unique_idents)]
+    attrs: BTreeSet<String>,
+    other: Option<bool>,
+}
+
+#[test]
+fn succeeds_on_no_attrs() {
+    let di = Receiver::from_derive_input(&parse_quote! {
+        struct Demo;
+    })
+    .unwrap();
+
+    assert!(di.attrs.is_empty());
+}
+
+#[test]
+fn succeeds_on_valid_input() {
+    let di = Receiver::from_derive_input(&parse_quote! {
+        #[allow(dead_code)]
+        /// testing
+        #[another]
+        struct Demo;
+    })
+    .unwrap();
+
+    assert_eq!(di.attrs.len(), 3);
+    assert!(di.attrs.contains("allow"));
+    assert!(di.attrs.contains("another"));
+    assert!(di.attrs.contains("doc"));
+    assert_eq!(di.other, None);
+}
+
+#[test]
+fn errors_combined_with_others() {
+    let e = Receiver::from_derive_input(&parse_quote! {
+        #[path::to::attr(dead_code)]
+        #[a(other = 5)]
+        struct Demo;
+    })
+    .map(|_| "Should have failed")
+    .unwrap_err();
+
+    let error = e.to_string();
+
+    assert_eq!(e.len(), 2);
+
+    // Look for the error on the field `other`
+    assert!(error.contains("at other"));
+
+    // Look for the invalid path from attrs conversion
+    assert!(error.contains("`path::to::attr`"));
+}