feat(strings): add IUtf8ToAsciiConverter and ICharacterMappingLoader interfaces

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Co-Authored-By: Claude <noreply@anthropic.com>

src/Umbraco.Core/Strings/ICharacterMappingLoader.cs

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+using System.Collections.Frozen;
+
+namespace Umbraco.Cms.Core.Strings;
+
+/// <summary>
+/// Loads character mappings from JSON files.
+/// </summary>
+public interface ICharacterMappingLoader
+{
+    /// <summary>
+    /// Loads all mapping files and returns combined FrozenDictionary.
+    /// Higher priority mappings override lower priority.
+    /// </summary>
+    /// <returns>Frozen dictionary of character to string mappings.</returns>
+    FrozenDictionary<char, string> LoadMappings();
+}

src/Umbraco.Core/Strings/IUtf8ToAsciiConverter.cs

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+namespace Umbraco.Cms.Core.Strings;
+
+/// <summary>
+/// Converts UTF-8 text to ASCII, handling accented characters and transliteration.
+/// </summary>
+public interface IUtf8ToAsciiConverter
+{
+    /// <summary>
+    /// Converts text to ASCII, returning a new string.
+    /// </summary>
+    /// <param name="text">The text to convert.</param>
+    /// <param name="fallback">Character to use for unmappable characters. Default '?'.</param>
+    /// <returns>The ASCII-converted string.</returns>
+    string Convert(string? text, char fallback = '?');
+
+    /// <summary>
+    /// Converts text to ASCII, writing to output span.
+    /// Zero-allocation for callers who provide buffer.
+    /// </summary>
+    /// <param name="input">The input text span.</param>
+    /// <param name="output">The output buffer. Must be at least input.Length * 4.</param>
+    /// <param name="fallback">Character to use for unmappable characters. Default '?'.</param>
+    /// <returns>Number of characters written to output.</returns>
+    int Convert(ReadOnlySpan<char> input, Span<char> output, char fallback = '?');
+}

tests/Umbraco.Tests.UnitTests/Umbraco.Core/Strings/Utf8ToAsciiConverterInterfaceTests.cs

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+using NUnit.Framework;
+using Umbraco.Cms.Core.Strings;
+
+namespace Umbraco.Cms.Tests.UnitTests.Umbraco.Core.Strings;
+
+[TestFixture]
+public class Utf8ToAsciiConverterInterfaceTests
+{
+    [Test]
+    public void IUtf8ToAsciiConverter_HasConvertStringMethod()
+    {
+        var type = typeof(IUtf8ToAsciiConverter);
+        var method = type.GetMethod("Convert", new[] { typeof(string), typeof(char) });
+
+        Assert.IsNotNull(method);
+        Assert.AreEqual(typeof(string), method.ReturnType);
+    }
+
+    [Test]
+    public void IUtf8ToAsciiConverter_HasConvertSpanMethod()
+    {
+        var type = typeof(IUtf8ToAsciiConverter);
+        var methods = type.GetMethods().Where(m => m.Name == "Convert").ToList();
+
+        Assert.That(methods.Count, Is.GreaterThanOrEqualTo(2), "Should have at least 2 Convert overloads");
+    }
+}