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/symmetric_difference.rs

@@ -0,0 +1,110 @@
+use super::*;
+
+use crate::Flags;
+
+#[test]
+fn cases() {
+    case(
+        TestFlags::empty(),
+        &[
+            (TestFlags::empty(), 0),
+            (TestFlags::all(), 1 | 1 << 1 | 1 << 2),
+            (TestFlags::from_bits_retain(1 << 3), 1 << 3),
+        ],
+        TestFlags::symmetric_difference,
+        TestFlags::toggle,
+    );
+
+    case(
+        TestFlags::A,
+        &[
+            (TestFlags::empty(), 1),
+            (TestFlags::A, 0),
+            (TestFlags::all(), 1 << 1 | 1 << 2),
+        ],
+        TestFlags::symmetric_difference,
+        TestFlags::toggle,
+    );
+
+    case(
+        TestFlags::A | TestFlags::B | TestFlags::from_bits_retain(1 << 3),
+        &[
+            (TestFlags::ABC, 1 << 2 | 1 << 3),
+            (TestFlags::from_bits_retain(1 << 3), 1 | 1 << 1),
+        ],
+        TestFlags::symmetric_difference,
+        TestFlags::toggle,
+    );
+}
+
+#[track_caller]
+fn case<T: Flags + std::fmt::Debug + std::ops::BitXor<Output = T> + std::ops::BitXorAssign + Copy>(
+    value: T,
+    inputs: &[(T, T::Bits)],
+    mut inherent_sym_diff: impl FnMut(T, T) -> T,
+    mut inherent_toggle: impl FnMut(&mut T, T),
+) where
+    T::Bits: std::fmt::Debug + PartialEq + Copy,
+{
+    for (input, expected) in inputs {
+        assert_eq!(
+            *expected,
+            inherent_sym_diff(value, *input).bits(),
+            "{:?}.symmetric_difference({:?})",
+            value,
+            input
+        );
+        assert_eq!(
+            *expected,
+            Flags::symmetric_difference(value, *input).bits(),
+            "Flags::symmetric_difference({:?}, {:?})",
+            value,
+            input
+        );
+        assert_eq!(
+            *expected,
+            (value ^ *input).bits(),
+            "{:?} ^ {:?}",
+            value,
+            input
+        );
+        assert_eq!(
+            *expected,
+            {
+                let mut value = value;
+                value ^= *input;
+                value
+            }
+            .bits(),
+            "{:?} ^= {:?}",
+            value,
+            input,
+        );
+
+        assert_eq!(
+            *expected,
+            {
+                let mut value = value;
+                inherent_toggle(&mut value, *input);
+                value
+            }
+            .bits(),
+            "{:?}.toggle({:?})",
+            value,
+            input,
+        );
+
+        assert_eq!(
+            *expected,
+            {
+                let mut value = value;
+                Flags::toggle(&mut value, *input);
+                value
+            }
+            .bits(),
+            "{:?}.toggle({:?})",
+            value,
+            input,
+        );
+    }
+}