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/contracts/test_integration.py

@@ -14,6 +14,8 @@ from unittest.mock import MagicMock, patch
 
 import numpy as np
 
+from abogen.domain.enums import Language
+
 from abogen.tts_plugin.engine import Engine, EngineSession
 from abogen.tts_plugin.errors import EngineError
 from abogen.tts_plugin.plugin_manager import PluginManager, get_plugin_manager, reset_plugin_manager
@@ -328,7 +330,7 @@ class TestRegression:
         manager._loaded = True
         
         with patch("abogen.tts_plugin.utils.get_plugin_manager", return_value=manager):
-            backend = create_pipeline("mock_tts", lang_code="a", device="cpu")
+            backend = create_pipeline("mock_tts", language=Language.EN_US, device="cpu")
             
             # Old interface: pipeline(text, voice=..., speed=..., split_pattern=...)
             segments = list(backend(

tests/contracts/test_plugin_manager_contract.py

@@ -3,6 +3,7 @@
 import pytest
 from unittest.mock import MagicMock, patch
 
+from abogen.domain.enums import Language
 from abogen.tts_plugin.plugin_manager import PluginManager, get_plugin_manager, reset_plugin_manager
 from abogen.tts_plugin.utils import Pipeline, create_pipeline
 from abogen.tts_plugin.engine import Engine, EngineSession
@@ -175,7 +176,7 @@ class TestCreatePipelineCompat:
             mock_engine = FakeEngine()
             mock_manager.create_engine.return_value = mock_engine
             
-            backend = create_pipeline("kokoro", lang_code="a", device="cpu")
+            backend = create_pipeline("kokoro", language=Language.EN_US, device="cpu")
             
             assert callable(backend)
             mock_manager.create_engine.assert_called_once()
@@ -185,7 +186,7 @@ class TestCreatePipelineCompat:
             assert call_args.kwargs["model_path"] is None
             assert isinstance(call_args.kwargs["config"], EngineConfig)
             assert call_args.kwargs["config"].device == "cpu"
-            assert call_args.kwargs["config"].lang_code == "a"
+            assert call_args.kwargs["config"].language == Language.EN_US
     
     def test_create_pipeline_raises_for_unknown_plugin(self):
         """create_pipeline raises KeyError for unknown plugins."""

tests/contracts/test_types_contract.py

@@ -8,6 +8,7 @@ These tests verify that value objects satisfy the architectural requirements:
 
 import pytest
 
+from abogen.domain.enums import Language
 from abogen.tts_plugin.types import (
     AudioFormat,
     Duration,
@@ -192,23 +193,23 @@ class TestEngineConfigContract:
         config = EngineConfig(device="cuda:0")
         assert config.device == "cuda:0"
 
-    def test_default_lang_code(self) -> None:
+    def test_default_language(self) -> None:
         config = EngineConfig()
-        assert config.lang_code == "a"
+        assert config.language == Language.EN_US
 
-    def test_custom_lang_code(self) -> None:
-        config = EngineConfig(lang_code="j")
-        assert config.lang_code == "j"
+    def test_custom_language(self) -> None:
+        config = EngineConfig(language=Language.JA)
+        assert config.language == Language.JA
 
     def test_immutability(self) -> None:
         config = EngineConfig()
         with pytest.raises(AttributeError):
             config.device = "cuda:0"  # type: ignore[misc]
 
-    def test_immutability_lang_code(self) -> None:
+    def test_immutability_language(self) -> None:
         config = EngineConfig()
         with pytest.raises(AttributeError):
-            config.lang_code = "j"  # type: ignore[misc]
+            config.language = Language.JA  # type: ignore[misc]
 
     def test_unknown_keys_ignored_per_spec(self) -> None:
         """Architecture spec: Unknown keys are ignored (no error).
@@ -225,11 +226,11 @@ class TestEngineConfigContract:
         EngineConfig may contain fields that are not relevant to every plugin.
         Plugins MUST ignore fields they do not need, not raise on them.
         """
-        config = EngineConfig(device="cuda:0", lang_code="j")
+        config = EngineConfig(device="cuda:0", language=Language.JA)
         assert config.device == "cuda:0"
-        assert config.lang_code == "j"
+        assert config.language == Language.JA
         # A plugin that only needs device simply reads config.device
-        # and ignores config.lang_code — this must not raise.
+        # and ignores config.language — this must not raise.
 
     def test_engine_config_contains_engine_instance_configuration(self) -> None:
         """Architecture Amendment #1: EngineConfig definition.
@@ -238,7 +239,7 @@ class TestEngineConfigContract:
         Engine instance is created and that remain constant throughout
         the lifetime of that Engine.
         """
-        config = EngineConfig(device="cpu", lang_code="a")
+        config = EngineConfig(device="cpu", language=Language.EN_US)
         # Both fields are init-time, immutable, engine-scoped.
         assert config.device == "cpu"
-        assert config.lang_code == "a"
+        assert config.language == Language.EN_US