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/defaults.rs

@@ -0,0 +1,189 @@
+use darling::FromDeriveInput;
+use syn::parse_quote;
+
+mod foo {
+    pub mod bar {
+        pub fn init() -> String {
+            String::from("hello")
+        }
+    }
+}
+
+#[derive(FromDeriveInput)]
+#[darling(attributes(speak))]
+pub struct SpeakerOpts {
+    #[darling(default = foo::bar::init)]
+    first_word: String,
+}
+
+#[test]
+fn path_default() {
+    let speaker: SpeakerOpts = FromDeriveInput::from_derive_input(&parse_quote! {
+        struct Foo;
+    })
+    .expect("Unit struct with no attrs should parse");
+
+    assert_eq!(speaker.first_word, "hello");
+}
+
+/// Tests in this module capture the somewhat-confusing behavior observed when defaults
+/// are set at both the field and container level.
+///
+/// The general rule is that more-specific declarations preempt less-specific ones; this is
+/// unsurprising and allows for granular control over what happens when parsing an AST.
+mod stacked_defaults {
+    use darling::{FromDeriveInput, FromMeta};
+    use syn::parse_quote;
+
+    fn jane() -> String {
+        "Jane".into()
+    }
+
+    #[derive(FromMeta)]
+    #[darling(default)]
+    struct PersonName {
+        #[darling(default = "jane")]
+        first: String,
+        #[darling(default)]
+        middle: String,
+        last: String,
+    }
+
+    impl Default for PersonName {
+        fn default() -> Self {
+            Self {
+                first: "John".into(),
+                middle: "T".into(),
+                last: "Doe".into(),
+            }
+        }
+    }
+
+    #[derive(FromDeriveInput)]
+    #[darling(attributes(person))]
+    struct Person {
+        #[darling(default)]
+        name: PersonName,
+        age: u8,
+    }
+
+    #[test]
+    fn name_first_only() {
+        let person = Person::from_derive_input(&parse_quote! {
+            #[person(name(first = "Bill"), age = 5)]
+            struct Foo;
+        })
+        .unwrap();
+
+        assert_eq!(person.name.first, "Bill");
+        assert_eq!(
+            person.name.middle, "",
+            "Explicit field-level default should preempt container-level default"
+        );
+        assert_eq!(
+            person.name.last, "Doe",
+            "Absence of a field-level default falls back to container-level default"
+        );
+    }
+
+    /// This is the most surprising case. The presence of `name()` means we invoke
+    /// `PersonName::from_list(&[])`. When that finishes parsing each of the zero nested
+    /// items it has received, it will then start filling in missing fields, using the
+    /// explicit field-level defaults for `first` and `middle`, while for `last` it will
+    /// use the `last` field from the container-level default.
+    #[test]
+    fn name_empty_list() {
+        let person = Person::from_derive_input(&parse_quote! {
+            #[person(name(), age = 5)]
+            struct Foo;
+        })
+        .unwrap();
+
+        assert_eq!(person.name.first, "Jane");
+        assert_eq!(person.name.middle, "");
+        assert_eq!(person.name.last, "Doe");
+    }
+
+    #[test]
+    fn no_name() {
+        let person = Person::from_derive_input(&parse_quote! {
+            #[person(age = 5)]
+            struct Foo;
+        })
+        .unwrap();
+
+        assert_eq!(person.age, 5);
+        assert_eq!(
+            person.name.first, "John",
+            "If `name` is not specified, `Person`'s field-level default should be used"
+        );
+        assert_eq!(person.name.middle, "T");
+        assert_eq!(person.name.last, "Doe");
+    }
+}
+
+mod implicit_default {
+    use darling::{util::Flag, FromDeriveInput};
+    use syn::parse_quote;
+
+    // No use of `darling(default)` here at all!
+    // This struct will fill in missing fields using FromMeta::from_none.
+    #[derive(FromDeriveInput)]
+    #[darling(attributes(person))]
+    struct Person {
+        first_name: String,
+        last_name: Option<String>,
+        lefty: Flag,
+    }
+
+    #[test]
+    fn missing_fields_fill() {
+        let person = Person::from_derive_input(&parse_quote! {
+            #[person(first_name = "James")]
+            struct Foo;
+        })
+        .unwrap();
+
+        assert_eq!(person.first_name, "James");
+        assert_eq!(person.last_name, None);
+        assert!(!person.lefty.is_present());
+    }
+}
+
+/// Test that a field-level implicit default using FromMeta::from_none is superseded
+/// by the parent declaring `#[darling(default)]`.
+mod overridden_implicit_default {
+    use darling::{util::Flag, FromDeriveInput};
+    use syn::parse_quote;
+
+    #[derive(FromDeriveInput)]
+    #[darling(default, attributes(person))]
+    struct Person {
+        first_name: String,
+        last_name: Option<String>,
+        lefty: Flag,
+    }
+
+    impl Default for Person {
+        fn default() -> Self {
+            Self {
+                first_name: "Jane".into(),
+                last_name: Some("Doe".into()),
+                lefty: Flag::default(),
+            }
+        }
+    }
+
+    #[test]
+    fn fill_missing() {
+        let person = Person::from_derive_input(&parse_quote!(
+            #[person(last_name = "Archer")]
+            struct Foo;
+        ))
+        .unwrap();
+
+        assert_eq!(person.first_name, "Jane");
+        assert_eq!(person.last_name, Some("Archer".into()));
+        assert!(!person.lefty.is_present());
+    }
+}