refactor: unify Language enum across all layers

- EngineConfig.language: Language (was lang_code: str = 'a')
- Engine owns _KOKORO_LANG_MAP, engine_language(), supported_languages()
- Engine provides language_for_voice_id() for voice catalog
- Plugins/kokoro/__init__.py calls engine_language() internally
- create_pipeline(plugin_id, language=Language) — no kokoro codes
- pipeline_factory.py clean of kokoro-specific code
- Domain functions raise TypeError if non-enum passed
- WebUI api.py: _parse_language() helper at API boundary
- Voice catalog returns ISO codes (lang.value)
- Constants: LANGUAGE_DESCRIPTIONS keyed by Language enum
- All tests updated for Language enum
- 1414 tests pass

tests/test_split_pattern.py

@@ -1,10 +1,11 @@
-"""Tests for split pattern logic (3 identical copies in codebase)."""
+"""Tests for split pattern logic."""
 import os
 import sys
 sys.path.insert(0, os.path.abspath(os.path.join(os.path.dirname(__file__), "..")))
 
 import pytest
 
+from abogen.domain.enums import Language
 from abogen.domain.split_pattern import get_split_pattern
 
 
@@ -12,49 +13,49 @@ from abogen.domain.split_pattern import get_split_pattern
 
 class TestEnglish:
     def test_english_sentence(self):
-        assert get_split_pattern("en-US", "Sentence") == "\n"
+        assert get_split_pattern(Language.EN_US, "Sentence") == "\n"
 
     def test_english_sentence_comma(self):
-        assert get_split_pattern("en-US", "Sentence + Comma") == "\n"
+        assert get_split_pattern(Language.EN_US, "Sentence + Comma") == "\n"
 
     def test_english_line(self):
-        assert get_split_pattern("en-US", "Line") == "\n"
+        assert get_split_pattern(Language.EN_US, "Line") == "\n"
 
     def test_english_disabled(self):
-        assert get_split_pattern("en-US", "Disabled") == "\n"
+        assert get_split_pattern(Language.EN_US, "Disabled") == "\n"
 
     def test_english_gb(self):
-        assert get_split_pattern("en-GB", "Sentence") == "\n"
+        assert get_split_pattern(Language.EN_GB, "Sentence") == "\n"
 
 
 # --- CJK languages ---
 
 class TestCJK:
     def test_chinese_disabled(self):
-        pattern = get_split_pattern("zh", "Disabled")
+        pattern = get_split_pattern(Language.ZH, "Disabled")
         assert pattern != "\n"
         assert r"\n+" in pattern
 
     def test_chinese_line(self):
-        pattern = get_split_pattern("zh", "Line")
+        pattern = get_split_pattern(Language.ZH, "Line")
         assert pattern != "\n"
         assert r"\n+" in pattern
 
     def test_chinese_sentence(self):
-        pattern = get_split_pattern("zh", "Sentence")
+        pattern = get_split_pattern(Language.ZH, "Sentence")
         assert r"\n+" in pattern
 
     def test_chinese_sentence_comma(self):
-        pattern = get_split_pattern("zh", "Sentence + Comma")
+        pattern = get_split_pattern(Language.ZH, "Sentence + Comma")
         assert r"\n+" in pattern
 
     def test_japanese_disabled(self):
-        pattern = get_split_pattern("ja", "Disabled")
+        pattern = get_split_pattern(Language.JA, "Disabled")
         assert pattern != "\n"
         assert r"\n+" in pattern
 
     def test_japanese_sentence(self):
-        pattern = get_split_pattern("ja", "Sentence")
+        pattern = get_split_pattern(Language.JA, "Sentence")
         assert r"\n+" in pattern
 
 
@@ -62,22 +63,17 @@ class TestCJK:
 
 class TestOtherLanguages:
     def test_spanish_sentence(self):
-        pattern = get_split_pattern("es", "Sentence")
+        pattern = get_split_pattern(Language.ES, "Sentence")
         assert r"\n+" in pattern
 
     def test_spanish_line(self):
-        assert get_split_pattern("es", "Line") == "\n"
+        assert get_split_pattern(Language.ES, "Line") == "\n"
 
     def test_spanish_disabled(self):
-        # canonical: \n+ for non-CJK Disabled
-        assert get_split_pattern("es", "Disabled") == r"\n+"
+        assert get_split_pattern(Language.ES, "Disabled") == r"\n+"
 
     def test_french_sentence_comma(self):
-        pattern = get_split_pattern("fr", "Sentence + Comma")
-        assert r"\n+" in pattern
-
-    def test_unknown_lang(self):
-        pattern = get_split_pattern("x", "Sentence")
+        pattern = get_split_pattern(Language.FR, "Sentence + Comma")
         assert r"\n+" in pattern
 
 
@@ -85,17 +81,17 @@ class TestOtherLanguages:
 
 class TestPatternStructure:
     def test_sentence_has_lookbehind(self):
-        pattern = get_split_pattern("es", "Sentence")
+        pattern = get_split_pattern(Language.ES, "Sentence")
         assert r"(?<=" in pattern
 
     def test_sentence_comma_has_comma_chars(self):
-        pattern = get_split_pattern("es", "Sentence + Comma")
+        pattern = get_split_pattern(Language.ES, "Sentence + Comma")
         assert "," in pattern
 
     def test_cjk_spacing_uses_star(self):
-        pattern = get_split_pattern("zh", "Sentence")
+        pattern = get_split_pattern(Language.ZH, "Sentence")
         assert r"\s*" in pattern
 
     def test_non_cjk_spacing_uses_plus(self):
-        pattern = get_split_pattern("es", "Sentence")
+        pattern = get_split_pattern(Language.ES, "Sentence")
         assert r"\s+" in pattern