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/bitflags/src/tests/is_empty.rs

@@ -0,0 +1,31 @@
+use super::*;
+
+use crate::Flags;
+
+#[test]
+fn cases() {
+    case(true, TestFlags::empty(), TestFlags::is_empty);
+
+    case(false, TestFlags::A, TestFlags::is_empty);
+    case(false, TestFlags::ABC, TestFlags::is_empty);
+    case(
+        false,
+        TestFlags::from_bits_retain(1 << 3),
+        TestFlags::is_empty,
+    );
+
+    case(true, TestZero::empty(), TestZero::is_empty);
+
+    case(true, TestEmpty::empty(), TestEmpty::is_empty);
+}
+
+#[track_caller]
+fn case<T: Flags + std::fmt::Debug>(expected: bool, value: T, inherent: impl FnOnce(&T) -> bool) {
+    assert_eq!(expected, inherent(&value), "{:?}.is_empty()", value);
+    assert_eq!(
+        expected,
+        Flags::is_empty(&value),
+        "Flags::is_empty({:?})",
+        value
+    );
+}