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/split_declaration.rs

@@ -0,0 +1,67 @@
+//! When input is split across multiple attributes on one element,
+//! darling should collapse that into one struct.
+
+use darling::{Error, FromDeriveInput};
+use syn::parse_quote;
+
+#[derive(Debug, FromDeriveInput, PartialEq, Eq)]
+#[darling(attributes(split))]
+struct Lorem {
+    foo: String,
+    bar: bool,
+}
+
+#[test]
+fn split_attributes_accrue_to_instance() {
+    let di = parse_quote! {
+        #[split(foo = "Hello")]
+        #[split(bar)]
+        pub struct Foo;
+    };
+
+    let parsed = Lorem::from_derive_input(&di).unwrap();
+    assert_eq!(
+        parsed,
+        Lorem {
+            foo: "Hello".to_string(),
+            bar: true,
+        }
+    );
+}
+
+#[test]
+fn duplicates_across_split_attrs_error() {
+    let di = parse_quote! {
+        #[split(foo = "Hello")]
+        #[split(foo = "World", bar)]
+        pub struct Foo;
+    };
+
+    let pr = Lorem::from_derive_input(&di).unwrap_err();
+    assert!(pr.has_span());
+    assert_eq!(pr.to_string(), Error::duplicate_field("foo").to_string());
+}
+
+#[test]
+fn multiple_errors_accrue_to_instance() {
+    let di = parse_quote! {
+        #[split(foo = "Hello")]
+        #[split(foo = "World")]
+        pub struct Foo;
+    };
+
+    let pr = Lorem::from_derive_input(&di);
+    let err: Error = pr.unwrap_err();
+    assert_eq!(2, err.len());
+    let mut errs = err.into_iter().peekable();
+    assert_eq!(
+        errs.peek().unwrap().to_string(),
+        Error::duplicate_field("foo").to_string()
+    );
+    assert!(errs.next().unwrap().has_span());
+    assert_eq!(
+        errs.next().unwrap().to_string(),
+        Error::missing_field("bar").to_string()
+    );
+    assert!(errs.next().is_none());
+}