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/unicode-escape.json

@@ -0,0 +1,8 @@
+{
+    "answer1": {"type": "string", "value": "\u000B"},
+    "answer4": {"type": "string", "value": "\u03B4α"},
+    "answer8": {"type": "string", "value": "\u03B4β"},
+    "answer9": {"type": "string", "value": "\uc0de"},
+    "answer10": {"type": "string", "value": "\u03B4α"},
+    "answer11": {"type": "string", "value": "\uABC1"}
+}