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/truncate.rs

@@ -0,0 +1,29 @@
+use super::*;
+
+use crate::Flags;
+
+#[test]
+fn cases() {
+    case(
+        TestFlags::ABC | TestFlags::from_bits_retain(1 << 3),
+        TestFlags::ABC,
+    );
+
+    case(TestZero::empty(), TestZero::empty());
+
+    case(TestZero::all(), TestZero::all());
+
+    case(
+        TestFlags::from_bits_retain(1 << 3) | TestFlags::all(),
+        TestFlags::all(),
+    );
+}
+
+#[track_caller]
+fn case<T: Flags + std::fmt::Debug>(mut before: T, after: T)
+where
+    T: std::fmt::Debug + PartialEq + Copy,
+{
+    before.truncate();
+    assert_eq!(before, after, "{:?}.truncate()", before);
+}