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/string-escapes.toml

@@ -0,0 +1,14 @@
+backspace = "This string has a \b backspace character."
+tab = "This string has a \t tab character."
+newline = "This string has a \n new line character."
+formfeed = "This string has a \f form feed character."
+carriage = "This string has a \r carriage return character."
+quote = "This string has a \" quote character."
+slash = "This string has a / slash character."
+backslash = "This string has a \\ backslash character."
+notunicode1 = "This string does not have a unicode \\u escape."
+notunicode2 = "This string does not have a unicode \u005Cu escape."
+notunicode3 = "This string does not have a unicode \\u0075 escape."
+notunicode4 = "This string does not have a unicode \\\u0075 escape."
+delete = "This string has a \u007F delete control code."
+unitseparator = "This string has a \u001F unit separator control code."