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/rwtext.x

@@ -0,0 +1,22 @@
+SECTIONS {
+  .rwtext : ALIGN(4)
+  {
+    . = ALIGN (4);
+    *(.rwtext.literal .rwtext .rwtext.literal.* .rwtext.*)
+    . = ALIGN(4);
+  } > RWTEXT
+
+  .rwtext.wifi :
+  {
+    . = ALIGN(4);
+    *( .wifi0iram  .wifi0iram.*)
+    *( .wifirxiram  .wifirxiram.*)
+    *( .wifislprxiram  .wifislprxiram.*)
+    *( .wifislpiram  .wifislpiram.*)
+    *( .phyiram  .phyiram.*)
+    *( .iram1  .iram1.*)
+    *( .wifiextrairam.* )
+    *( .coexiram.* )
+    . = ALIGN(4);
+  } > RWTEXT AT > RODATA
+}