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/esp32c6/rom-functions.x

@@ -0,0 +1,13 @@
+INCLUDE "rom/esp32c6.rom.api.ld"
+INCLUDE "rom/esp32c6.rom.coexist.ld"
+INCLUDE "rom/esp32c6.rom.heap.ld"
+INCLUDE "rom/esp32c6.rom.ld"
+INCLUDE "rom/esp32c6.rom.libgcc.ld"
+INCLUDE "rom/esp32c6.rom.net80211.ld"
+INCLUDE "rom/esp32c6.rom.phy.ld"
+INCLUDE "rom/esp32c6.rom.pp.ld"
+INCLUDE "rom/esp32c6.rom.rvfp.ld"
+INCLUDE "rom/esp32c6.rom.spiflash.ld"
+INCLUDE "rom/esp32c6.rom.version.ld"
+
+INCLUDE "rom/additional.ld"