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/tests/compile-fail/with_closure_capture.rs

@@ -0,0 +1,21 @@
+use darling::{FromDeriveInput, FromMeta};
+
+#[derive(FromDeriveInput)]
+#[darling(attributes(demo))]
+pub struct Receiver {
+    example1: String,
+    #[darling(
+        // This should fail because `example1` is a local that's been captured
+        // from the `FromDeriveInput` impl. That's disallowed because exposing
+        // those internals would make any change to the derived method body a
+        // potentially-breaking change.
+        with = |m| Ok(
+            String::from_meta(m)?.to_uppercase()
+            + example1.1.as_ref().map(|s| s.as_str()).unwrap_or("")
+        ),
+        default
+    )]
+    example2: String,
+}
+
+fn main() {}