🟢 Port problem 709 (To Lower Case) from Clojure to C++23

solutions/0701-0800/0709-to-lower-case/README.md

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+# 709. To Lower Case
+
+[LeetCode problem 709](https://leetcode.com/problems/to-lower-case/)
+
+- **Difficulty**: Easy
+- **Tags**: string
+
+## Description
+
+Given a string `s`, return the string after converting every uppercase
+letter to lowercase.

solutions/0701-0800/0709-to-lower-case/solution.cpp

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+#include "solution.hpp"
+
+#include <algorithm>
+#include <cctype>
+
+namespace leetcode::p0709 {
+
+std::string solve(std::string s) {
+    std::ranges::transform(
+        s, s.begin(), [](unsigned char c) { return static_cast<char>(std::tolower(c)); });
+    return s;
+}
+
+} // namespace leetcode::p0709

solutions/0701-0800/0709-to-lower-case/solution.hpp

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+#pragma once
+
+#include <string>
+
+namespace leetcode::p0709 {
+
+// optimized: O(n) time, O(n) space — map each character through
+// std::tolower.
+std::string solve(std::string s);
+
+} // namespace leetcode::p0709

solutions/0701-0800/0709-to-lower-case/test.cpp

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+#include <catch2/catch_test_macros.hpp>
+
+#include "solution.hpp"
+
+TEST_CASE("Problem 709", "[p0709]") {
+    CHECK(leetcode::p0709::solve("HELLO") == "hello");
+    CHECK(leetcode::p0709::solve("world") == "world");
+    CHECK(leetcode::p0709::solve("CloJure") == "clojure");
+    CHECK(leetcode::p0709::solve("") == "");
+    CHECK(leetcode::p0709::solve("123!@#") == "123!@#");
+}