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/esp32c2/rom/additional.ld

@@ -0,0 +1,17 @@
+memset = 0x40000488;
+memcpy = 0x4000048c;
+memmove = 0x40000490;
+memcmp = 0x40000494;
+
+strcpy = 0x40000498;
+strncpy = 0x4000049c;
+strncmp = 0x400004a4;
+
+PROVIDE ( strcat = 0x4000050c );
+PROVIDE ( strcmp = 0x400004a0 );
+PROVIDE ( strchr = 0x40000514 );
+PROVIDE ( strlcpy = 0x40000524 );
+PROVIDE ( strstr = 0x400004ac );
+PROVIDE ( strcasecmp = 0x40000504 );
+PROVIDE ( strdup = 0x40000510 );
+PROVIDE ( atoi = 0x40000580 );