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/syn/tests/test_token_trees.rs

@@ -0,0 +1,38 @@
+#![allow(
+    clippy::elidable_lifetime_names,
+    clippy::needless_lifetimes,
+    clippy::uninlined_format_args
+)]
+
+#[macro_use]
+mod snapshot;
+
+mod debug;
+
+use proc_macro2::TokenStream;
+use quote::quote;
+use syn::Lit;
+
+#[test]
+fn test_struct() {
+    let input = "
+        #[derive(Debug, Clone)]
+        pub struct Item {
+            pub ident: Ident,
+            pub attrs: Vec<Attribute>,
+        }
+    ";
+
+    snapshot!(input as TokenStream, @r##"
+    TokenStream(
+        `# [derive (Debug , Clone)] pub struct Item { pub ident : Ident , pub attrs : Vec < Attribute >, }`,
+    )
+    "##);
+}
+
+#[test]
+fn test_literal_mangling() {
+    let code = "0_4";
+    let parsed: Lit = syn::parse_str(code).unwrap();
+    assert_eq!(code, quote!(#parsed).to_string());
+}