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/from_variant.rs

@@ -0,0 +1,57 @@
+use darling::FromVariant;
+use syn::{spanned::Spanned, Expr, ExprLit, LitInt};
+
+#[derive(FromVariant)]
+#[darling(from_ident, attributes(hello))]
+#[allow(dead_code)]
+pub struct Lorem {
+    ident: syn::Ident,
+    into: Option<bool>,
+    skip: Option<bool>,
+    discriminant: Option<syn::Expr>,
+    fields: darling::ast::Fields<syn::Type>,
+}
+
+impl From<syn::Ident> for Lorem {
+    fn from(ident: syn::Ident) -> Self {
+        Lorem {
+            ident,
+            into: Default::default(),
+            skip: Default::default(),
+            discriminant: None,
+            fields: darling::ast::Style::Unit.into(),
+        }
+    }
+}
+
+#[test]
+fn discriminant() {
+    let input: syn::DeriveInput = syn::parse_str(
+        r#"
+    pub enum Test {
+        Works = 1,
+        AlsoWorks = 2,
+    }
+    "#,
+    )
+    .unwrap();
+
+    let span = input.span();
+    if let syn::Data::Enum(enm) = input.data {
+        let lorem = Lorem::from_variant(
+            enm.variants
+                .first()
+                .expect("Hardcoded input has one variant"),
+        )
+        .expect("FromVariant can process the discriminant");
+        assert_eq!(
+            lorem.discriminant,
+            Some(Expr::Lit(ExprLit {
+                attrs: vec![],
+                lit: LitInt::new("1", span).into(),
+            }))
+        )
+    } else {
+        panic!("Data should be enum");
+    }
+}