Add LowercaseFormatter with proper UTF-8 support

The new LowercaseFormatter provides reliable UTF-8 aware lowercase
conversion for international text, ensuring accented characters and
Turkish special cases (İ/i) are handled correctly using mb_strtolower().

This formatter complements UppercaseFormatter and is essential for
applications requiring proper internationalization support when
manipulating text in various languages including those with special
character mapping rules.

Includes comprehensive tests covering ASCII, Latin accents, Turkish
characters, non-Latin scripts, emoji, combining diacritics,
right-to-left text, multi-byte characters, and mixed content.

Assisted-by: OpenCode (GLM-4.7)

Author: henriquemoody

README.md

@@ -64,6 +64,7 @@ See the [PlaceholderFormatter documentation](docs/PlaceholderFormatter.md) and [
 | [ImperialAreaFormatter](docs/ImperialAreaFormatter.md)         | Imperial area promotion (in², ft², yd², ac, mi²)                 |
 | [ImperialLengthFormatter](docs/ImperialLengthFormatter.md)     | Imperial length promotion (in, ft, yd, mi)                       |
 | [ImperialMassFormatter](docs/ImperialMassFormatter.md)         | Imperial mass promotion (oz, lb, st, ton)                        |
+| [LowercaseFormatter](docs/LowercaseFormatter.md)               | Convert string to lowercase                                      |
 | [MaskFormatter](docs/MaskFormatter.md)                         | Range-based string masking with Unicode support                  |
 | [MassFormatter](docs/MassFormatter.md)                         | Metric mass promotion (mg, g, kg, t)                             |
 | [MetricFormatter](docs/MetricFormatter.md)                     | Metric length promotion (mm, cm, m, km)                          |

docs/LowercaseFormatter.md

@@ -0,0 +1,88 @@
+<!--
+SPDX-FileCopyrightText: (c) Respect Project Contributors
+SPDX-License-Identifier: ISC
+SPDX-FileContributor: Henrique Moody <henriquemoody@gmail.com>
+-->
+
+# LowercaseFormatter
+
+The `LowercaseFormatter` converts strings to lowercase with proper UTF-8 character support for international text.
+
+## Usage
+
+### Basic Usage
+
+```php
+use Respect\StringFormatter\LowercaseFormatter;
+
+$formatter = new LowercaseFormatter();
+
+echo $formatter->format('HELLO WORLD');
+// Outputs: "hello world"
+```
+
+### Unicode Characters
+
+```php
+use Respect\StringFormatter\LowercaseFormatter;
+
+$formatter = new LowercaseFormatter();
+
+echo $formatter->format('CAFÉ FRANÇAIS');
+// Outputs: "café français"
+
+echo $formatter->format('コンニチハ');
+// Outputs: "コンニチハ"
+```
+
+### Mixed Content
+
+```php
+use Respect\StringFormatter\LowercaseFormatter;
+
+$formatter = new LowercaseFormatter();
+
+echo $formatter->format('HELLO WORLD 😊');
+// Outputs: "hello world 😊"
+```
+
+## API
+
+### `LowercaseFormatter::__construct`
+
+- `__construct()`
+
+Creates a new lowercase formatter instance.
+
+### `format`
+
+- `format(string $input): string`
+
+Converts the input string to lowercase using UTF-8 aware conversion.
+
+**Parameters:**
+
+- `$input`: The string to convert to lowercase
+
+**Returns:** The lowercase string
+
+## Examples
+
+| Input        | Output       | Description                   |
+| ------------ | ------------ | ----------------------------- |
+| `HELLO`      | `hello`      | Simple ASCII text             |
+| `CAFÉ`       | `café`       | Latin characters with accents |
+| `ПРИВЕТ`     | `привет`     | Cyrillic text                 |
+| `コンニチハ` | `コンニチハ` | Japanese text                 |
+| `HELLO 😊`   | `hello 😊`   | Text with emoji               |
+| `ÉÎÔÛ`       | `éîôû`       | Multiple accented characters  |
+
+## Notes
+
+- Uses `mb_strtolower()` for proper Unicode handling
+- Preserves accent marks and diacritical marks
+- Works with all Unicode scripts (Latin, Cyrillic, Greek, CJK, etc.)
+- Emoji and special symbols are preserved unchanged
+- Combining diacritics are properly handled
+- Numbers and special characters remain unchanged
+- Empty strings return empty strings

src/LowercaseFormatter.php

@@ -0,0 +1,21 @@
+<?php
+
+/*
+ * SPDX-FileCopyrightText: (c) Respect Project Contributors
+ * SPDX-License-Identifier: ISC
+ * SPDX-FileContributor: Henrique Moody <henriquemoody@gmail.com>
+ */
+
+declare(strict_types=1);
+
+namespace Respect\StringFormatter;
+
+use function mb_strtolower;
+
+final readonly class LowercaseFormatter implements Formatter
+{
+    public function format(string $input): string
+    {
+        return mb_strtolower($input);
+    }
+}

src/Mixin/Builder.php

@@ -30,6 +30,8 @@ public static function imperialMass(string $unit): FormatterBuilder;
 
     public static function date(string $format = 'Y-m-d H:i:s'): FormatterBuilder;
 
+    public static function lowercase(): FormatterBuilder;
+
     public static function mask(string $range, string $replacement = '*'): FormatterBuilder;
 
     public static function metric(string $unit): FormatterBuilder;

src/Mixin/Chain.php

@@ -30,6 +30,8 @@ public function imperialMass(string $unit): FormatterBuilder;
 
     public function date(string $format = 'Y-m-d H:i:s'): FormatterBuilder;
 
+    public function lowercase(): FormatterBuilder;
+
     public function mask(string $range, string $replacement = '*'): FormatterBuilder;
 
     public function metric(string $unit): FormatterBuilder;