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_asyncness.rs

@@ -0,0 +1,49 @@
+#![allow(
+    clippy::elidable_lifetime_names,
+    clippy::needless_lifetimes,
+    clippy::uninlined_format_args
+)]
+
+#[macro_use]
+mod snapshot;
+
+mod debug;
+
+use syn::{Expr, Item};
+
+#[test]
+fn test_async_fn() {
+    let input = "async fn process() {}";
+
+    snapshot!(input as Item, @r#"
+    Item::Fn {
+        vis: Visibility::Inherited,
+        sig: Signature {
+            asyncness: Some,
+            ident: "process",
+            generics: Generics,
+            output: ReturnType::Default,
+        },
+        block: Block {
+            stmts: [],
+        },
+    }
+    "#);
+}
+
+#[test]
+fn test_async_closure() {
+    let input = "async || {}";
+
+    snapshot!(input as Expr, @r#"
+    Expr::Closure {
+        asyncness: Some,
+        output: ReturnType::Default,
+        body: Expr::Block {
+            block: Block {
+                stmts: [],
+            },
+        },
+    }
+    "#);
+}