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/esp32s2/rom/additional.ld

@@ -0,0 +1,20 @@
+memcmp = 0x4001ab40;
+memcpy = 0x4001aba8;
+memmove = 0x4001acb0;
+memset = 0x4001ad3c;
+
+strcpy = 0x40007cfc;
+strncpy = 0x40007f20;
+strncmp = 0x4001ae64;
+
+bzero = 0x400078c8;
+
+PROVIDE ( cache_dbus_mmu_set = 0x40018eb0 );
+
+PROVIDE ( strcat = 0x4001ad90 );
+PROVIDE ( strcmp = 0x40007be4 );
+PROVIDE ( strchr = 0x4001adb0 );
+PROVIDE ( strlcpy = 0x4001adf8 );
+PROVIDE ( strstr = 0x4001aee8 );
+PROVIDE ( strcasecmp = 0x40007b38 );
+PROVIDE ( strdup = 0x40007d84 );