feat(lex_parser): 初始化词法解析器模块

新增词法解析器库 `smcc_lex_parser`，包含基础的词法规则解析功能：
- 支持字符、字符串、数字、标识符的解析
- 支持跳过注释、空白符、行尾等辅助函数
- 提供对应的单元测试用例，覆盖各类合法与非法输入情况

该模块依赖 `libcore`，并被 `smcc_lex` 模块引用以支持更上层的词法分析逻辑。

libs/lex_parser/tests/test_string.c

@@ -0,0 +1,62 @@
+// test_string.c
+#include <lex_parser.h>
+#include <utest/acutest.h>
+
+cbool check_string(const char *str, const char *expect, cstring_t *output) {
+    log_set_level(&__default_logger_root, 0);
+    core_pos_t pos = core_pos_init();
+    core_mem_stream_t mem_stream;
+    core_stream_t *stream =
+        core_mem_stream_init(&mem_stream, str, smcc_strlen(str), false);
+
+    cbool ret = lex_parse_string(stream, &pos, output);
+    if (ret && expect) {
+        return strcmp(output->data, expect) == 0;
+    }
+    return ret;
+}
+
+#define CHECK_STRING_VALID(str, expect)                                        \
+    do {                                                                       \
+        cstring_t _output = cstring_new();                                     \
+        cbool ret = check_string(str, expect, &_output);                       \
+        TEST_CHECK(ret == true);                                               \
+        TEST_CHECK(strcmp(_output.data, expect) == 0);                         \
+        cstring_free(&_output);                                                \
+    } while (0)
+
+#define CHECK_STRING_INVALID(str)                                              \
+    do {                                                                       \
+        cstring_t _output = cstring_new();                                     \
+        cbool ret = check_string(str, NULL, &_output);                         \
+        TEST_CHECK(ret == false);                                              \
+        cstring_free(&_output);                                                \
+    } while (0)
+
+void test_simple_string(void) {
+    TEST_CASE("simple strings");
+    CHECK_STRING_VALID("\"\"", "");
+    CHECK_STRING_VALID("\"hello\"", "hello");
+    CHECK_STRING_VALID("\"hello world\"", "hello world");
+}
+
+void test_escape_string(void) {
+    TEST_CASE("escape strings");
+    CHECK_STRING_VALID("\"\\n\"", "\n");
+    CHECK_STRING_VALID("\"\\t\"", "\t");
+    CHECK_STRING_VALID("\"\\\"\"", "\"");
+    CHECK_STRING_VALID("\"Hello\\nWorld\"", "Hello\nWorld");
+}
+
+void test_invalid_string(void) {
+    TEST_CASE("invalid strings");
+    CHECK_STRING_INVALID("\"unterminated");
+    CHECK_STRING_INVALID("\"newline\n\"");
+}
+
+TEST_LIST = {
+    {"test_simple_string", test_simple_string},
+    {"test_escape_string", test_escape_string},
+    {"test_invalid_string", test_invalid_string},
+    {NULL, NULL},
+};