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/complement.rs

@@ -0,0 +1,53 @@
+use super::*;
+
+use crate::Flags;
+
+#[test]
+fn cases() {
+    case(0, TestFlags::all(), TestFlags::complement);
+    case(0, TestFlags::from_bits_retain(!0), TestFlags::complement);
+
+    case(1 | 1 << 1, TestFlags::C, TestFlags::complement);
+    case(
+        1 | 1 << 1,
+        TestFlags::C | TestFlags::from_bits_retain(1 << 3),
+        TestFlags::complement,
+    );
+
+    case(
+        1 | 1 << 1 | 1 << 2,
+        TestFlags::empty(),
+        TestFlags::complement,
+    );
+    case(
+        1 | 1 << 1 | 1 << 2,
+        TestFlags::from_bits_retain(1 << 3),
+        TestFlags::complement,
+    );
+
+    case(0, TestZero::empty(), TestZero::complement);
+
+    case(0, TestEmpty::empty(), TestEmpty::complement);
+
+    case(1 << 2, TestOverlapping::AB, TestOverlapping::complement);
+
+    case(!0, TestExternal::empty(), TestExternal::complement);
+}
+
+#[track_caller]
+fn case<T: Flags + std::fmt::Debug + std::ops::Not<Output = T> + Copy>(
+    expected: T::Bits,
+    value: T,
+    inherent: impl FnOnce(T) -> T,
+) where
+    T::Bits: std::fmt::Debug + PartialEq,
+{
+    assert_eq!(expected, inherent(value).bits(), "{:?}.complement()", value);
+    assert_eq!(
+        expected,
+        Flags::complement(value).bits(),
+        "Flags::complement({:?})",
+        value
+    );
+    assert_eq!(expected, (!value).bits(), "!{:?}", value);
+}