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/basic-toml/tests/valid/example.json

@@ -0,0 +1,14 @@
+{
+  "best-day-ever": {"type": "datetime", "value": "1987-07-05T17:45:00Z"},
+  "numtheory": {
+    "boring": {"type": "bool", "value": "false"},
+    "perfection": {
+      "type": "array",
+      "value": [
+        {"type": "integer", "value": "6"},
+        {"type": "integer", "value": "28"},
+        {"type": "integer", "value": "496"}
+      ]
+    }
+  }
+}