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/pin-project-lite/tests/expand/multifields/enum.rs

@@ -0,0 +1,20 @@
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+use pin_project_lite::pin_project;
+
+pin_project! {
+#[project_replace = EnumProjReplace]
+enum Enum<T, U> {
+    Struct {
+        #[pin]
+        pinned1: T,
+        #[pin]
+        pinned2: T,
+        unpinned1: U,
+        unpinned2: U,
+    },
+    Unit,
+}
+}
+
+fn main() {}