feat(strings): add complete character mappings from golden test data

Adds missing character mappings to ensure behavioral equivalence with
original Utf8ToAsciiConverter implementation. Creates extended-mappings.json
with 1,213 additional characters covering punctuation, symbols, extended
Latin, Greek, and other Unicode blocks.

Also fixes 8 Cyrillic character mappings to match original behavior.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/Umbraco.Core/Strings/CharacterMappingLoader.cs

@@ -11,7 +11,7 @@ namespace Umbraco.Cms.Core.Strings;
 public sealed class CharacterMappingLoader : ICharacterMappingLoader
 {
     private static readonly string[] BuiltInFiles =
-        ["ligatures.json", "special-latin.json", "cyrillic.json"];
+        ["ligatures.json", "special-latin.json", "cyrillic.json", "extended-mappings.json"];
 
     private static readonly JsonSerializerOptions JsonOptions = new()
     {

src/Umbraco.Core/Strings/CharacterMappings/cyrillic.json

@@ -15,16 +15,16 @@
     "д": "d",
     "Е": "E",
     "е": "e",
-    "Ё": "Yo",
-    "ё": "yo",
+    "Ё": "E",
+    "ё": "e",
     "Ж": "Zh",
     "ж": "zh",
     "З": "Z",
     "з": "z",
     "И": "I",
     "и": "i",
-    "Й": "Y",
-    "й": "y",
+    "Й": "I",
+    "й": "i",
     "К": "K",
     "к": "k",
     "Л": "L",
@@ -49,14 +49,14 @@
     "ф": "f",
     "Х": "Kh",
     "х": "kh",
-    "Ц": "Ts",
-    "ц": "ts",
+    "Ц": "F",
+    "ц": "f",
     "Ч": "Ch",
     "ч": "ch",
     "Ш": "Sh",
     "ш": "sh",
-    "Щ": "Shch",
-    "щ": "shch",
+    "Щ": "Sh",
+    "щ": "sh",
     "Ъ": "\"",
     "ъ": "\"",
     "Ы": "Y",