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/rustversion/tests/test_eval.rs

@@ -0,0 +1,20 @@
+#[rustversion::any(
+    stable,
+    stable(1.34),
+    stable(1.34.0),
+    beta,
+    nightly,
+    nightly(2020-02-25),
+    since(1.34),
+    since(2020-02-25),
+    before(1.34),
+    before(2020-02-25),
+    not(nightly),
+    all(stable, beta, nightly),
+)]
+fn success() {}
+
+#[test]
+fn test() {
+    success();
+}