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/rwdata.x

@@ -0,0 +1,63 @@
+
+
+SECTIONS {
+  .data : ALIGN(4)
+  {
+    _data_start = ABSOLUTE(.);
+    . = ALIGN (4);
+
+    #IF ESP_HAL_PLACE_SWITCH_TABLES_IN_RAM
+      *(.rodata.*_esp_hal_internal_handler*)
+      *(.rodata..Lswitch.table.*)
+      *(.rodata.cst*)
+    #ENDIF
+
+    #IF ESP_HAL_PLACE_ANON_IN_RAM
+      *(.rodata..Lanon .rodata..Lanon.*)
+    #ENDIF
+
+    *(.sdata .sdata.* .sdata2 .sdata2.*);
+    *(.data .data.*);
+    *(.data1)
+    _data_end = ABSOLUTE(.);
+    . = ALIGN(4);
+  } > RWDATA AT > RODATA
+
+  /* LMA of .data */
+  _sidata = LOADADDR(.data);
+
+  .bss (NOLOAD) : ALIGN(4)
+  {
+    _bss_start = ABSOLUTE(.);
+    . = ALIGN (4);
+    *(.dynsbss)
+    *(.sbss)
+    *(.sbss.*)
+    *(.gnu.linkonce.sb.*)
+    *(.scommon)
+    *(.sbss2)
+    *(.sbss2.*)
+    *(.gnu.linkonce.sb2.*)
+    *(.dynbss)
+    *(.sbss .sbss.* .bss .bss.*);
+    *(.share.mem)
+    *(.gnu.linkonce.b.*)
+    *(COMMON)
+    _bss_end = ABSOLUTE(.);
+    . = ALIGN(4);
+  } > RWDATA
+
+  .noinit (NOLOAD) : ALIGN(4)
+  {
+    . = ALIGN(4);
+    *(.noinit .noinit.*)
+    . = ALIGN(4);
+  } > RWDATA
+
+  .data.wifi :
+  {
+    . = ALIGN(4);
+    *( .dram1 .dram1.*)
+    . = ALIGN(4);
+  } > RWDATA AT > RODATA
+}