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_macro/src/lib.rs

@@ -0,0 +1,40 @@
+use darling_core::{derive, Error};
+use proc_macro::TokenStream;
+use syn::parse_macro_input;
+
+#[proc_macro_derive(FromMeta, attributes(darling))]
+pub fn derive_from_meta(input: TokenStream) -> TokenStream {
+    derive::from_meta(&parse_macro_input!(input)).into()
+}
+
+#[proc_macro_derive(FromMetaItem, attributes(darling))]
+pub fn derive_from_meta_item(_input: TokenStream) -> TokenStream {
+    Error::custom("darling::FromMetaItem has been replaced by darling::FromMeta")
+        .write_errors()
+        .into()
+}
+
+#[proc_macro_derive(FromAttributes, attributes(darling))]
+pub fn derive_from_attributes(input: TokenStream) -> TokenStream {
+    derive::from_attributes(&parse_macro_input!(input)).into()
+}
+
+#[proc_macro_derive(FromDeriveInput, attributes(darling))]
+pub fn derive_from_input(input: TokenStream) -> TokenStream {
+    derive::from_derive_input(&parse_macro_input!(input)).into()
+}
+
+#[proc_macro_derive(FromField, attributes(darling))]
+pub fn derive_field(input: TokenStream) -> TokenStream {
+    derive::from_field(&parse_macro_input!(input)).into()
+}
+
+#[proc_macro_derive(FromTypeParam, attributes(darling))]
+pub fn derive_type_param(input: TokenStream) -> TokenStream {
+    derive::from_type_param(&parse_macro_input!(input)).into()
+}
+
+#[proc_macro_derive(FromVariant, attributes(darling))]
+pub fn derive_variant(input: TokenStream) -> TokenStream {
+    derive::from_variant(&parse_macro_input!(input)).into()
+}