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/error.rs

@@ -0,0 +1,54 @@
+//! In case of bad input, parsing should fail. The error should have locations set in derived implementations.
+
+// The use of fields in debug print commands does not count as "used",
+// which causes the fields to trigger an unwanted dead code warning.
+#![allow(dead_code)]
+
+use darling::{FromDeriveInput, FromMeta};
+use syn::parse_quote;
+
+#[derive(Debug, FromMeta)]
+struct Dolor {
+    #[darling(rename = "amet")]
+    sit: bool,
+    world: bool,
+}
+
+#[derive(Debug, FromDeriveInput)]
+#[darling(from_ident, attributes(hello))]
+struct Lorem {
+    ident: syn::Ident,
+    ipsum: Dolor,
+}
+
+impl From<syn::Ident> for Lorem {
+    fn from(ident: syn::Ident) -> Self {
+        Lorem {
+            ident,
+            ipsum: Dolor {
+                sit: false,
+                world: true,
+            },
+        }
+    }
+}
+
+#[test]
+fn parsing_fail() {
+    let di = parse_quote! {
+        #[hello(ipsum(amet = "yes", world = false))]
+        pub struct Foo;
+    };
+
+    println!("{}", Lorem::from_derive_input(&di).unwrap_err());
+}
+
+#[test]
+fn missing_field() {
+    let di = parse_quote! {
+        #[hello(ipsum(amet = true))]
+        pub struct Foo;
+    };
+
+    println!("{}", Lorem::from_derive_input(&di).unwrap_err());
+}