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/sections/rodata.x

@@ -0,0 +1,20 @@
+
+
+SECTIONS {
+  .rodata : ALIGN(4)
+  {
+    . = ALIGN (4);
+    _rodata_start = ABSOLUTE(.);
+    *(.rodata .rodata.*)
+    *(.srodata .srodata.*)
+    . = ALIGN(4);
+    _rodata_end = ABSOLUTE(.);
+  } > RODATA
+
+  .rodata.wifi : ALIGN(4)
+  {
+    . = ALIGN(4);
+    *( .rodata_wlog_*.* )
+    . = ALIGN(4);
+  } > RODATA
+}