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/strum_macros/src/helpers/mod.rs

@@ -0,0 +1,42 @@
+pub use self::case_style::snakify;
+pub use self::inner_variant_props::HasInnerVariantProperties;
+pub use self::type_props::HasTypeProperties;
+pub use self::variant_props::HasStrumVariantProperties;
+
+pub mod case_style;
+pub mod inner_variant_props;
+mod metadata;
+pub mod type_props;
+pub mod variant_props;
+
+use proc_macro2::Span;
+use quote::ToTokens;
+use syn::spanned::Spanned;
+
+pub fn non_enum_error() -> syn::Error {
+    syn::Error::new(Span::call_site(), "This macro only supports enums.")
+}
+
+pub fn non_unit_variant_error() -> syn::Error {
+    syn::Error::new(
+        Span::call_site(),
+        "This macro only supports enums of strictly unit variants. Consider \
+        using it in conjunction with [`EnumDiscriminants`]",
+    )
+}
+
+pub fn strum_discriminants_passthrough_error(span: &impl Spanned) -> syn::Error {
+    syn::Error::new(
+        span.span(),
+        "expected a pass-through attribute, e.g. #[strum_discriminants(serde(rename = \"var0\"))]",
+    )
+}
+
+pub fn occurrence_error<T: ToTokens>(fst: T, snd: T, attr: &str) -> syn::Error {
+    let mut e = syn::Error::new_spanned(
+        snd,
+        format!("Found multiple occurrences of strum({})", attr),
+    );
+    e.combine(syn::Error::new_spanned(fst, "first one here"));
+    e
+}