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/enumset/tests/bitset_explicit_repr.rs

@@ -0,0 +1,31 @@
+#![deny(warnings)]
+
+use enumset::*;
+
+#[derive(EnumSetType, Debug)]
+#[enumset(repr = "u16")]
+enum ReprEnum {
+    A,
+    B,
+    C,
+    D,
+    E,
+    F,
+    G,
+    H,
+}
+
+const _: () = assert!(EnumSet::<ReprEnum>::all().as_repr() == 0b11111111);
+
+#[test]
+fn test() {
+    let mut set = EnumSet::<ReprEnum>::new();
+    set.insert(ReprEnum::B);
+    set.insert(ReprEnum::F);
+
+    let repr: u16 = set.as_repr();
+    assert_eq!((1 << 1) | (1 << 5), repr,);
+
+    let set2 = unsafe { EnumSet::<ReprEnum>::from_repr_unchecked(repr) };
+    assert_eq!(set, set2);
+}