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/hard_example.json

@@ -0,0 +1 @@
+{"the":{"hard":{"another_test_string":{"type":"string","value":" Same thing, but with a string #"},"bit#":{"multi_line_array":{"type":"array","value":[{"type":"string","value":"]"}]},"what?":{"type":"string","value":"You don't think some user won't do that?"}},"harder_test_string":{"type":"string","value":" And when \"'s are in the string, along with # \""},"test_array":{"type":"array","value":[{"type":"string","value":"] "},{"type":"string","value":" # "}]},"test_array2":{"type":"array","value":[{"type":"string","value":"Test #11 ]proved that"},{"type":"string","value":"Experiment #9 was a success"}]}},"test_string":{"type":"string","value":"You'll hate me after this - #"}}}