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/esp32c3/rom/additional.ld

@@ -0,0 +1,21 @@
+memset = 0x40000354;
+memcpy = 0x40000358;
+memmove = 0x4000035c;
+memcmp = 0x40000360;
+
+strncmp = 0x40000370;
+strncpy = 0x40000368;
+strcpy = 0x40000364;
+
+abs = 0x40000424;
+
+PROVIDE(cache_dbus_mmu_set = 0x40000564);
+
+PROVIDE( strcat = 0x400003d8 );
+PROVIDE( strcmp = 0x4000036c );
+PROVIDE( strchr = 0x400003e0 );
+PROVIDE( strlcpy = 0x400003f0 );
+PROVIDE( strstr = 0x40000378 );
+PROVIDE( strcasecmp = 0x400003d0 );
+PROVIDE( strdup = 0x400003dc );
+PROVIDE( atoi = 0x4000044c );