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-0.20.11/tests/meta_with.rs

@@ -0,0 +1,33 @@
+use darling::{util::parse_expr, FromDeriveInput, FromMeta};
+use syn::{parse_quote, Expr};
+
+#[derive(FromDeriveInput)]
+#[darling(attributes(demo))]
+pub struct Receiver {
+    #[darling(with = parse_expr::preserve_str_literal, map = Some)]
+    example1: Option<Expr>,
+    #[darling(
+        with = |m| Ok(String::from_meta(m)?.to_uppercase()),
+        map = Some
+    )]
+    example2: Option<String>,
+    // This is deliberately strange - it keeps the field name, and ignores
+    // the rest of the attribute. In normal operation, this is strongly discouraged.
+    // It's used here to verify that the parameter type is known even if it can't be
+    // inferred from usage within the closure.
+    #[darling(with = |m| Ok(m.path().clone()))]
+    example3: syn::Path,
+}
+
+#[test]
+fn handles_all_cases() {
+    let input = Receiver::from_derive_input(&parse_quote! {
+        #[demo(example1 = test::path, example2 = "hello", example3)]
+        struct Example;
+    })
+    .unwrap();
+
+    assert_eq!(input.example1, Some(parse_quote!(test::path)));
+    assert_eq!(input.example2, Some("HELLO".to_string()));
+    assert_eq!(input.example3, parse_quote!(example3));
+}