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/esp32s2/esp32s2.x

@@ -0,0 +1,42 @@
+
+/* before memory.x to allow override */
+ENTRY(ESP32Reset)
+
+/* after memory.x to allow override */
+PROVIDE(__pre_init = DefaultPreInit);
+PROVIDE(__zero_bss = default_mem_hook);
+PROVIDE(__init_data = default_mem_hook);
+PROVIDE(__post_init = default_post_init);
+
+PROVIDE(__level_1_interrupt = handle_interrupts);
+PROVIDE(__level_2_interrupt = handle_interrupts);
+PROVIDE(__level_3_interrupt = handle_interrupts);
+
+INCLUDE exception.x
+
+/* Fixups for esp32s2 */
+SECTIONS {
+  .rwdata_dummy (NOLOAD) : ALIGN(4)
+  {
+    . = ORIGIN(RWDATA) + SIZEOF(.rwtext) + SIZEOF(.rwtext.wifi);
+  } > RWDATA
+}
+INSERT BEFORE .data;
+
+INCLUDE "fixups/rtc_fast_rwdata_dummy.x"
+/* End of fixups for esp32s2 */
+
+/* Shared sections - ordering matters */
+INCLUDE "rwtext.x"
+INCLUDE "text.x"
+INCLUDE "rwdata.x"
+INCLUDE "rodata.x"
+INCLUDE "rtc_fast.x"
+INCLUDE "rtc_slow.x"
+INCLUDE "stack.x"
+INCLUDE "dram2.x"
+/* End of Shared sections */
+
+EXTERN(DefaultHandler);
+
+INCLUDE "device.x"