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/esp32c6/rom/esp32c6.rom.coexist.ld

@@ -0,0 +1,47 @@
+/*
+ * SPDX-FileCopyrightText: 2022 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+/* ROM function interface esp32c6.rom.coexist.ld for esp32c6
+ *
+ *
+ * Generated from ./target/esp32c6/interface-esp32c6.yml md5sum 06c13e133e0743d09b87aba30d3e213b
+ *
+ * Compatible with ROM where ECO version equal or greater to 0.
+ *
+ * THIS FILE WAS AUTOMATICALLY GENERATED. DO NOT EDIT.
+ */
+
+/***************************************
+ Group rom_coexist
+ ***************************************/
+
+/* Functions */
+esp_coex_rom_version_get = 0x40000afc;
+coex_bt_release = 0x40000b00;
+coex_bt_request = 0x40000b04;
+coex_core_ble_conn_dyn_prio_get = 0x40000b08;
+/*coex_core_event_duration_get = 0x40000b0c;*/
+coex_core_pti_get = 0x40000b10;
+coex_core_release = 0x40000b14;
+coex_core_request = 0x40000b18;
+coex_core_status_get = 0x40000b1c;
+/*coex_core_timer_idx_get = 0x40000b20;*/
+coex_event_duration_get = 0x40000b24;
+coex_hw_timer_disable = 0x40000b28;
+coex_hw_timer_enable = 0x40000b2c;
+coex_hw_timer_set = 0x40000b30;
+coex_schm_interval_set = 0x40000b34;
+coex_schm_lock = 0x40000b38;
+coex_schm_unlock = 0x40000b3c;
+coex_status_get = 0x40000b40;
+coex_wifi_release = 0x40000b44;
+esp_coex_ble_conn_dynamic_prio_get = 0x40000b48;
+/* Data (.data, .bss, .rodata) */
+coex_env_ptr = 0x4087ffc4;
+coex_pti_tab_ptr = 0x4087ffc0;
+coex_schm_env_ptr = 0x4087ffbc;
+coexist_funcs = 0x4087ffb8;
+g_coa_funcs_p = 0x4087ffb4;
+g_coex_param_ptr = 0x4087ffb0;