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/raw-multiline-string.toml

@@ -0,0 +1,9 @@
+oneline = '''This string has a ' quote character.'''
+firstnl = '''
+This string has a ' quote character.'''
+multiline = '''
+This string
+has ' a quote character
+and more than
+one newline
+in it.'''