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-1.3.2/tests/basic.rs

@@ -0,0 +1,20 @@
+#![no_std]
+
+use bitflags::bitflags;
+
+bitflags! {
+    /// baz
+    struct Flags: u32 {
+        const A = 0b00000001;
+        #[doc = "bar"]
+        const B = 0b00000010;
+        const C = 0b00000100;
+        #[doc = "foo"]
+        const ABC = Flags::A.bits | Flags::B.bits | Flags::C.bits;
+    }
+}
+
+#[test]
+fn basic() {
+    assert_eq!(Flags::ABC, Flags::A | Flags::B | Flags::C);
+}