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/esp-hal/ld/riscv/hal-defaults.x

@@ -0,0 +1,35 @@
+PROVIDE(DefaultHandler = EspDefaultHandler);
+
+PROVIDE(interrupt1 = DefaultHandler);
+PROVIDE(interrupt2 = DefaultHandler);
+PROVIDE(interrupt3 = DefaultHandler);
+PROVIDE(interrupt4 = DefaultHandler);
+PROVIDE(interrupt5 = DefaultHandler);
+PROVIDE(interrupt6 = DefaultHandler);
+PROVIDE(interrupt7 = DefaultHandler);
+PROVIDE(interrupt8 = DefaultHandler);
+PROVIDE(interrupt9 = DefaultHandler);
+PROVIDE(interrupt10 = DefaultHandler);
+PROVIDE(interrupt11 = DefaultHandler);
+PROVIDE(interrupt12 = DefaultHandler);
+PROVIDE(interrupt13 = DefaultHandler);
+PROVIDE(interrupt14 = DefaultHandler);
+PROVIDE(interrupt15 = DefaultHandler);
+PROVIDE(interrupt16 = DefaultHandler);
+PROVIDE(interrupt17 = DefaultHandler);
+PROVIDE(interrupt18 = DefaultHandler);
+PROVIDE(interrupt19 = DefaultHandler);
+PROVIDE(interrupt20 = DefaultHandler);
+PROVIDE(interrupt21 = DefaultHandler);
+PROVIDE(interrupt22 = DefaultHandler);
+PROVIDE(interrupt23 = DefaultHandler);
+PROVIDE(interrupt24 = DefaultHandler);
+PROVIDE(interrupt25 = DefaultHandler);
+PROVIDE(interrupt26 = DefaultHandler);
+PROVIDE(interrupt27 = DefaultHandler);
+PROVIDE(interrupt28 = DefaultHandler);
+PROVIDE(interrupt29 = DefaultHandler);
+PROVIDE(interrupt30 = DefaultHandler);
+PROVIDE(interrupt31 = DefaultHandler);
+
+INCLUDE "device.x"