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/compile-pass/redefinition/stringify.rs

@@ -0,0 +1,19 @@
+use bitflags::bitflags;
+
+// Checks for possible errors caused by overriding names used by `bitflags!` internally.
+
+#[allow(unused_macros)]
+macro_rules! stringify {
+    ($($t:tt)*) => { "..." };
+}
+
+bitflags! {
+    struct Test: u8 {
+        const A = 1;
+    }
+}
+
+fn main() {
+    // Just make sure we don't call the redefined `stringify` macro
+    assert_eq!(format!("{:?}", Test::A), "A");
+}