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/float-exponent.json

@@ -0,0 +1,11 @@
+{
+    "lower": {"type": "float", "value": "300.0"},
+    "upper": {"type": "float", "value": "300.0"},
+    "neg": {"type": "float", "value": "0.03"},
+    "pos": {"type": "float", "value": "300.0"},
+    "zero": {"type": "float", "value": "3.0"},
+    "pointlower": {"type": "float", "value": "310.0"},
+    "pointupper": {"type": "float", "value": "310.0"},
+    "prefix-zero-exp": {"type": "float", "value": "1000000.0"},
+    "prefix-zero-exp-plus": {"type": "float", "value": "1000000.0"}
+}