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/supports.rs

@@ -0,0 +1,90 @@
+use darling::{ast, FromDeriveInput, FromVariant};
+
+#[derive(Debug, FromDeriveInput)]
+#[darling(attributes(from_variants), supports(enum_any))]
+pub struct Container {
+    // The second type parameter can be anything that implements FromField, since
+    // FromDeriveInput will produce an error if given a struct.
+    data: ast::Data<Variant, ()>,
+}
+
+#[derive(Default, Debug, FromVariant)]
+#[darling(default, attributes(from_variants), supports(newtype, unit))]
+pub struct Variant {
+    into: Option<bool>,
+    skip: Option<bool>,
+}
+
+#[derive(Debug, FromDeriveInput)]
+#[darling(attributes(from_struct), supports(struct_named))]
+pub struct StructContainer {
+    // The second type parameter can be anything that implements FromVariant, since
+    // FromDeriveInput will produce an error if given an enum.
+    data: ast::Data<(), syn::Field>,
+}
+
+mod source {
+    use syn::{parse_quote, DeriveInput};
+
+    pub fn newtype_enum() -> DeriveInput {
+        parse_quote! {
+            enum Hello {
+                World(bool),
+                String(String),
+            }
+        }
+    }
+
+    pub fn named_field_enum() -> DeriveInput {
+        parse_quote! {
+            enum Hello {
+                Foo(u16),
+                World {
+                    name: String
+                },
+            }
+        }
+    }
+
+    pub fn empty_enum() -> DeriveInput {
+        parse_quote! {
+            enum Hello {}
+        }
+    }
+
+    pub fn named_struct() -> DeriveInput {
+        parse_quote! {
+            struct Hello {
+                world: bool,
+            }
+        }
+    }
+
+    pub fn tuple_struct() -> DeriveInput {
+        parse_quote! { struct Hello(String, bool); }
+    }
+}
+
+#[test]
+fn enum_newtype_or_unit() {
+    // Should pass
+    let container = Container::from_derive_input(&source::newtype_enum()).unwrap();
+    assert!(container.data.is_enum());
+
+    // Should error
+    Container::from_derive_input(&source::named_field_enum()).unwrap_err();
+    Container::from_derive_input(&source::named_struct()).unwrap_err();
+}
+
+#[test]
+fn struct_named() {
+    // Should pass
+    let container = StructContainer::from_derive_input(&source::named_struct()).unwrap();
+    assert!(container.data.is_struct());
+
+    // Should fail
+    StructContainer::from_derive_input(&source::tuple_struct()).unwrap_err();
+    StructContainer::from_derive_input(&source::named_field_enum()).unwrap_err();
+    StructContainer::from_derive_input(&source::newtype_enum()).unwrap_err();
+    StructContainer::from_derive_input(&source::empty_enum()).unwrap_err();
+}