feat: 添加字符串工具函数（split, trim, substring, replace, contains, upper, lower, starts_with, ends_with）及相关测试用例

examples/practical.ast

@@ -180,3 +180,30 @@ fn is_palindrome(s) {
 
 print("is_palindrome('radar'):", is_palindrome("radar"))
 print("is_palindrome('hello'):", is_palindrome("hello"))
+
+
+// ============================================================================
+// 7. 字符串工具（split / trim / substring / replace）
+// ============================================================================
+
+print("=== string utils ===")
+
+let csv = "Alice,Bob,Carol,Dave"
+let parts = split(csv, ",")
+print("split CSV:", parts)
+print("names count:", len(parts))
+
+let padded = "  hello  "
+print("trim:", "'" + trim(padded) + "'")
+
+print("substring('hello', 1, 3):", substring("hello", 1, 3))
+
+let template = "Hello, NAME!"
+let greeting = replace(template, "NAME", "Aster")
+print("replace:", greeting)
+
+print("contains('hello', 'ell'):", contains("hello", "ell"))
+print("upper:", upper("hello"))
+print("lower:", lower("HELLO"))
+print("starts_with('hello', 'he'):", starts_with("hello", "he"))
+print("ends_with('hello', 'lo'):", ends_with("hello", "lo"))