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/esp32/rom/additional.ld

@@ -0,0 +1,20 @@
+memcmp = 0x4000c260;
+memcpy = 0x4000c2c8;
+memmove = 0x4000c3c0;
+memset = 0x4000c44c;
+
+PROVIDE ( strcpy = 0x400013ac );
+PROVIDE ( abs = 0x40056340 );
+PROVIDE ( strncpy = 0x400015d4 );
+PROVIDE ( strncmp = 0x4000c5f4 );
+
+PROVIDE ( bzero = 0x4000c1f4 );
+
+PROVIDE ( strcat = 0x4000c518 );
+PROVIDE ( strcmp = 0x40001274 );
+PROVIDE ( strchr = 0x4000c53c );
+PROVIDE ( strlcpy = 0x4000c584 );
+PROVIDE ( strstr = 0x4000c674 );
+PROVIDE ( strcasecmp = 0x400011cc );
+PROVIDE ( strdup = 0x4000143c );
+PROVIDE ( atoi = 0x400566c4 );