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/skip.rs

@@ -0,0 +1,74 @@
+//! Test that skipped fields are not read into structs when they appear in input.
+
+use darling::{FromDeriveInput, FromMeta};
+use syn::parse_quote;
+
+#[derive(Debug, PartialEq, Eq, FromDeriveInput)]
+#[darling(attributes(skip_test))]
+pub struct Lorem {
+    ipsum: String,
+
+    #[darling(skip)]
+    dolor: u8,
+}
+
+/// Verify variant-level and field-level skip work correctly for enums.
+#[derive(Debug, FromMeta)]
+pub enum Sit {
+    Amet(bool),
+
+    #[darling(skip)]
+    Foo {
+        hello: bool,
+    },
+
+    Bar {
+        hello: bool,
+        #[darling(skip)]
+        world: u8,
+    },
+}
+
+#[test]
+fn verify_skipped_field_not_required() {
+    let di = parse_quote! {
+        #[skip_test(ipsum = "Hello")]
+        struct Baz;
+    };
+
+    assert_eq!(
+        Lorem::from_derive_input(&di).unwrap(),
+        Lorem {
+            ipsum: "Hello".to_string(),
+            dolor: 0,
+        }
+    );
+}
+
+/// This test verifies that a skipped field will still prefer an explicit default
+/// over the default that would come from its field type. It would be incorrect for
+/// `Defaulting::from_derive_input` to fail here, and it would be wrong for the value
+/// of `dolor` to be `None`.
+#[test]
+fn verify_default_supersedes_from_none() {
+    fn default_dolor() -> Option<u8> {
+        Some(2)
+    }
+
+    #[derive(Debug, PartialEq, Eq, FromDeriveInput)]
+    #[darling(attributes(skip_test))]
+    pub struct Defaulting {
+        #[darling(skip, default = "default_dolor")]
+        dolor: Option<u8>,
+    }
+
+    let di = parse_quote! {
+        #[skip_test]
+        struct Baz;
+    };
+
+    assert_eq!(
+        Defaulting::from_derive_input(&di).unwrap(),
+        Defaulting { dolor: Some(2) }
+    )
+}