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/fixups/rodata_dummy.x

@@ -0,0 +1,27 @@
+
+
+
+SECTIONS {
+  .rodata_dummy (NOLOAD) :
+  {
+    /* This dummy section represents the .flash.text section but in RODATA.
+     * Thus, it must have its alignment and (at least) its size.
+     */
+
+    /* Start at the same alignment constraint than .flash.text */
+
+    . = ALIGN(ALIGNOF(.text));
+
+    /* Create an empty gap as big as .text section */
+
+    . = . + SIZEOF(.text);
+
+    /* Prepare the alignment of the section above. Few bytes (0x20) must be
+     * added for the mapping header.
+     */
+
+    . = ALIGN(0x10000) + 0x20;
+    _rodata_reserved_start = .;
+  } > RODATA
+}
+INSERT BEFORE .rodata;

vendor/esp-hal/ld/sections/fixups/rtc_fast_rwdata_dummy.x

@@ -0,0 +1,14 @@
+/*
+ This fix up is required when the RTC fast memory is split across two address spaces.
+ This fix up pads the _data bus_ address space by the size of the code accessed by the instruction bus.
+*/
+
+SECTIONS {
+  .rtc_fast.dummy (NOLOAD) :
+  {
+    _rtc_dummy_start = ABSOLUTE(.); /* needed to make section proper size */
+    . = . + SIZEOF(.rtc_fast.text);
+    _rtc_dummy_end = ABSOLUTE(.); /* needed to make section proper size */
+  } > RTC_FAST_RWDATA
+}
+INSERT BEFORE .rtc_fast.data;