Add firmware slices 1–2: skeleton and Wi-Fi provisioning

Implement ESP-IDF walking skeleton with SoftAP, health API, and host
tests, then Slice 2 ISecureStore/NvsSecureStore, STA join, POST
/api/wifi, and the provisioning web UI.

Co-authored-by: Cursor <cursoragent@cursor.com>

Software/components/drivers/si4684/CMakeLists.txt

@@ -0,0 +1,6 @@
+idf_component_register(
+    SRCS "src/component_stub.cpp"
+    INCLUDE_DIRS "include"
+)
+
+target_compile_features(${COMPONENT_LIB} PUBLIC cxx_std_23)

Software/components/drivers/si4684/src/component_stub.cpp

@@ -0,0 +1,33 @@
+/**
+ * @file    component_stub.cpp
+ * @brief   Si4684 driver component placeholder (Slice 4).
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+
+namespace si4684::detail {
+
+/**
+ * @brief    si4684ComponentLinked — ensures the driver component links.
+ *
+ * @dname    si4684ComponentLinked
+ * @return   n/a (type)
+ * @pubstate n/a
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+void si4684ComponentLinked() noexcept {}
+
+} // namespace si4684::detail