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/indexmap/tests/macros_full_path.rs

@@ -0,0 +1,19 @@
+#[test]
+fn test_create_map() {
+    let _m = indexmap::indexmap! {
+        1 => 2,
+        7 => 1,
+        2 => 2,
+        3 => 3,
+    };
+}
+
+#[test]
+fn test_create_set() {
+    let _s = indexmap::indexset! {
+        1,
+        7,
+        2,
+        3,
+    };
+}