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/arrays.toml

@@ -0,0 +1,8 @@
+ints = [1, 2, 3]
+floats = [1.1, 2.1, 3.1]
+strings = ["a", "b", "c"]
+dates = [
+  1987-07-05T17:45:00Z,
+  1979-05-27T07:32:00Z,
+  2006-06-01T11:00:00Z,
+]