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/rtc_fast.x

@@ -0,0 +1,36 @@
+
+
+SECTIONS {
+  .rtc_fast.text : {
+   . = ALIGN(4);
+   *(.rtc_fast.literal .rtc_fast.text .rtc_fast.literal.* .rtc_fast.text.*)
+   . = ALIGN(4);
+  } > RTC_FAST_RWTEXT AT > RODATA
+  
+  .rtc_fast.data :
+  {
+    . = ALIGN(4);
+    _rtc_fast_data_start = ABSOLUTE(.);
+    *(.rtc_fast.data .rtc_fast.data.*)
+    _rtc_fast_data_end = ABSOLUTE(.);
+    . = ALIGN(4);
+  } > RTC_FAST_RWDATA AT > RODATA
+
+ .rtc_fast.bss (NOLOAD) :
+  {
+    . = ALIGN(4);
+    _rtc_fast_bss_start = ABSOLUTE(.);
+    *(.rtc_fast.bss .rtc_fast.bss.*)
+    _rtc_fast_bss_end = ABSOLUTE(.);
+    . = ALIGN(4);
+  } > RTC_FAST_RWDATA
+
+ .rtc_fast.persistent (NOLOAD) :
+  {
+    . = ALIGN(4);
+    _rtc_fast_persistent_start = ABSOLUTE(.);
+    *(.rtc_fast.persistent .rtc_fast.persistent.*)
+    _rtc_fast_persistent_end = ABSOLUTE(.);
+    . = ALIGN(4);
+  } > RTC_FAST_RWDATA
+}