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/stack.x

@@ -0,0 +1,43 @@
+#IF flip-link
+/* no Xtensa chip is supported - so we can assume RISC-V */
+SECTIONS {
+  /* must be last segment using RWDATA */
+  .stack (NOLOAD) : ALIGN(4)
+  {
+    _stack_end = ABSOLUTE(.);
+    _stack_end_cpu0 = ABSOLUTE(.);
+
+    /* Since we cannot know how much the alignment padding of the sections will add we shrink the stack for "the worst case"
+    */
+    . = . + LENGTH(RWDATA) -  (SIZEOF(.trap) + SIZEOF(.rwtext) + SIZEOF(.rwtext.wifi) + SIZEOF(.data) + SIZEOF(.bss) + SIZEOF(.noinit) + SIZEOF(.data.wifi)) - 304;
+
+    . = ALIGN (4);
+    _stack_start = ABSOLUTE(.);
+    _stack_start_cpu0 = ABSOLUTE(.);
+  } > RWDATA
+}
+INSERT BEFORE .trap;
+
+#ELSE
+
+SECTIONS {
+  /* must be last segment using RWDATA */
+  .stack (NOLOAD) : ALIGN(4)
+  {
+    . = ALIGN (4);
+    _stack_end = ABSOLUTE(.);
+    _stack_end_cpu0 = ABSOLUTE(.);
+  } > RWDATA
+}
+
+PROVIDE(_stack_start = ORIGIN(RWDATA) + LENGTH(RWDATA));
+PROVIDE(_stack_start_cpu0 = ORIGIN(RWDATA) + LENGTH(RWDATA));
+
+#ENDIF
+
+/*
+Provide the stack_guard for `stack-protector`
+
+Ideally the offset should be configurable - should be done once we have https://github.com/esp-rs/esp-hal/issues/1111
+*/
+PROVIDE(__stack_chk_guard = _stack_end + 4096);