feat(cmakev2): Added import_lib_direct example for cmakev2

This commit adds a new example at
examples/build_system/cmakev2/features/import_lib_direct to
demonstrate the cmakev2 ability to integrate with external CMake
projects easily.

examples/build_system/cmakev2/features/import_lib_direct/CMakeLists.txt

@@ -0,0 +1,11 @@
+# This example demonstrates a cmakev2 capability of importing an external
+# C library via FetchContent, built by its own CMake, and linked directly
+# to an IDF component without any IDF component wrapper for the library.
+
+cmake_minimum_required(VERSION 3.22)
+
+include($ENV{IDF_PATH}/tools/cmakev2/idf.cmake)
+
+project(import_lib_direct C CXX ASM)
+
+idf_project_default()

examples/build_system/cmakev2/features/import_lib_direct/README.md

@@ -0,0 +1,61 @@
+| Supported Targets | ESP32 | ESP32-C2 | ESP32-C3 | ESP32-C5 | ESP32-C6 | ESP32-C61 | ESP32-H2 | ESP32-H21 | ESP32-H4 | ESP32-P4 | ESP32-S2 | ESP32-S3 | ESP32-S31 |
+| ----------------- | ----- | -------- | -------- | -------- | -------- | --------- | -------- | --------- | -------- | -------- | -------- | -------- | --------- |
+
+# Import External C Library Directly (cmakev2)
+
+This example demonstrates importing an external C library that is a pure CMake project. It is downloaded at configure time, built by its own CMake, and linked directly to the IDF app without wrapping it as an IDF component.
+
+## Overview
+
+In Build System v1, integrating third-party CMake libraries required wrapping them as IDF components (see the `import_lib` example). With cmakev2, CMake's `FetchContent` module can be used to fetch and build an external CMake library (lwjson) and link the library to an IDF component using `target_link_libraries`.
+
+This example uses [lwjson](https://github.com/MaJerle/lwjson), a lightweight JSON parser for embedded systems.
+
+## Project Structure
+
+```
+import_lib_direct/
+├── CMakeLists.txt          # Initialize the IDF project
+├── README.md
+└── main/
+    ├── CMakeLists.txt      # FetchContent for lwjson, register component, link
+    └── main.c              # Uses lwjson API
+```
+
+## How It Works
+
+1. The **main component's CMakeLists.txt** uses `FetchContent_Declare()` and `FetchContent_MakeAvailable()` to download and build lwjson at configure time. The fetched sources land in `./build/_deps/`.
+
+2. After registering the main omponent with `idf_component_register`, it links the fetched library:
+   ```cmake
+   target_link_libraries(${COMPONENT_LIB} PUBLIC lwjson)
+   ```
+
+3. **main.c** uses the lwjson API (`lwjson_init`, `lwjson_parse`, `lwjson_find`, etc.) to parse a sample JSON string and print device name, cores, features, and specs.
+
+## How to Use
+
+Build and flash the example:
+
+```bash
+idf.py set-target <target>
+idf.py build
+idf.py flash monitor
+```
+
+## Expected Output
+
+```
+I (275) import_lib_direct: lwjson library imported directly (downloaded, built, linked) without IDF component wrapper
+I (275) import_lib_direct: Parsing JSON string...
+I (285) import_lib_direct: Device name: ESP32
+I (285) import_lib_direct: Number of cores: 2
+I (285) import_lib_direct: Features:
+I (295) import_lib_direct:   - WiFi
+I (295) import_lib_direct:   - Bluetooth
+I (295) import_lib_direct:   - GPIO
+I (305) import_lib_direct: Specifications:
+I (305) import_lib_direct:   Flash: 4MB
+I (305) import_lib_direct:   RAM: 520KB
+I (315) import_lib_direct: Example complete!
+```

examples/build_system/cmakev2/features/import_lib_direct/main/CMakeLists.txt

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+# Fetch the external lwjson library at configure time.
+include(FetchContent)
+fetchcontent_declare(
+    lwjson
+    GIT_REPOSITORY https://github.com/MaJerle/lwjson.git
+    GIT_TAG        v1.8.1
+    GIT_SHALLOW    TRUE
+)
+fetchcontent_makeavailable(lwjson)
+
+idf_component_register(SRCS "main.c"
+                       INCLUDE_DIRS ".")
+
+# Link the fetched lwjson library to this component
+target_link_libraries(${COMPONENT_LIB} PUBLIC lwjson)

examples/build_system/cmakev2/features/import_lib_direct/main/main.c

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+/*
+ * SPDX-FileCopyrightText: 2026 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Unlicense OR CC0-1.0
+ */
+
+/*
+ * This example demonstrates importing an external C library (lwjson)
+ * via FetchContent, built by its own CMake, and linked directly to this IDF component
+ * without any IDF component wrapper.
+ */
+
+#include <stdio.h>
+#include <string.h>
+#include "esp_log.h"
+#include "lwjson/lwjson.h"
+
+static const char *TAG = "import_lib_direct";
+
+#define LWJSON_TOKENS 64
+
+/* Sample JSON string to parse */
+static const char *sample_json =
+    "{"
+    "  \"name\": \"ESP32\","
+    "  \"cores\": 2,"
+    "  \"features\": [\"WiFi\", \"Bluetooth\", \"GPIO\"],"
+    "  \"specs\": {"
+    "    \"flash\": \"4MB\","
+    "    \"ram\": \"520KB\""
+    "  }"
+    "}";
+
+void app_main(void)
+{
+    ESP_LOGI(TAG, "lwjson library imported directly (downloaded, built, linked) without IDF component wrapper");
+
+    static lwjson_token_t tokens[LWJSON_TOKENS];
+    lwjson_t lwobj;
+
+    if (lwjson_init(&lwobj, tokens, LWJSON_TOKENS) != lwjsonOK) {
+        ESP_LOGE(TAG, "lwjson_init failed");
+        return;
+    }
+
+    if (lwjson_parse(&lwobj, sample_json) != lwjsonOK) {
+        ESP_LOGE(TAG, "lwjson_parse failed");
+        lwjson_free(&lwobj);
+        return;
+    }
+
+    ESP_LOGI(TAG, "Parsing JSON string...");
+
+    const lwjson_token_t *name_t = lwjson_find(&lwobj, "name");
+    if (name_t != NULL) {
+        size_t len;
+        const char *s = lwjson_get_val_string(name_t, &len);
+        if (s != NULL) {
+            ESP_LOGI(TAG, "Device name: %.*s", (int)len, s);
+        }
+    }
+
+    const lwjson_token_t *cores_t = lwjson_find(&lwobj, "cores");
+    if (cores_t != NULL && cores_t->type == LWJSON_TYPE_NUM_INT) {
+        ESP_LOGI(TAG, "Number of cores: %lld", (long long)lwjson_get_val_int(cores_t));
+    }
+
+    const lwjson_token_t *features_t = lwjson_find(&lwobj, "features");
+    if (features_t != NULL && features_t->type == LWJSON_TYPE_ARRAY) {
+        ESP_LOGI(TAG, "Features:");
+        const lwjson_token_t *child = lwjson_get_first_child(features_t);
+        while (child != NULL) {
+            if (child->type == LWJSON_TYPE_STRING) {
+                size_t len;
+                const char *s = lwjson_get_val_string(child, &len);
+                if (s != NULL) {
+                    ESP_LOGI(TAG, "  - %.*s", (int)len, s);
+                }
+            }
+            child = child->next;
+        }
+    }
+
+    const lwjson_token_t *flash_t = lwjson_find(&lwobj, "specs.flash");
+    const lwjson_token_t *ram_t = lwjson_find(&lwobj, "specs.ram");
+    if (flash_t != NULL || ram_t != NULL) {
+        ESP_LOGI(TAG, "Specifications:");
+        if (flash_t != NULL) {
+            size_t len;
+            const char *s = lwjson_get_val_string(flash_t, &len);
+            if (s != NULL) {
+                ESP_LOGI(TAG, "  Flash: %.*s", (int)len, s);
+            }
+        }
+        if (ram_t != NULL) {
+            size_t len;
+            const char *s = lwjson_get_val_string(ram_t, &len);
+            if (s != NULL) {
+                ESP_LOGI(TAG, "  RAM: %.*s", (int)len, s);
+            }
+        }
+    }
+
+    lwjson_free(&lwobj);
+
+    ESP_LOGI(TAG, "Example complete!");
+}