feat(tts-plugin): complete Plugin Architecture refactor

- Normalize Pipeline public API: create_pipeline(plugin_id, *, lang_code, device)
- EngineConfig: add lang_code field per Architecture Amendment #1
- Kokoro plugin reads config.lang_code (fixes functional regression)
- Static voice catalog in PluginManifest.voices (None = dynamic/VoiceLister)
- get_voices() reads from manifest without creating Engine
- Remove dead kwargs (sample_rate, auto_download, total_steps) from SuperTonic
- Clean up unused imports and dead code in engine implementations
- Fix test expectations for VoiceLister (mock overrides)
- Add clear_preview_pipelines() for resource management

tests/plugins/invalid_capabilities/__init__.py

@@ -1,29 +1,29 @@
-"""Invalid plugin: unknown capabilities."""
-
-from __future__ import annotations
-
-from pathlib import Path
-from typing import Any
-
-from abogen.tts_plugin.manifest import PluginManifest
-from abogen.tts_plugin.types import EngineConfig
-
-PLUGIN_MANIFEST = PluginManifest(
-    id="invalid_capabilities",
-    name="Invalid Capabilities Plugin",
-    version="1.0.0",
-    api_version="1.0",
-    description="Plugin with unknown capabilities",
-    author="Test Author",
-    capabilities=("voice_list", "unknown_capability", "another_unknown"),
-)
-
-MODEL_REQUIREMENTS: list[Any] = []
-
-
-def create_engine(
-    context: Any,
-    model_path: Path | None,
-    config: EngineConfig,
-) -> Any:
-    raise NotImplementedError("This plugin is invalid")
+"""Invalid plugin: unknown capabilities."""
+
+from __future__ import annotations
+
+from pathlib import Path
+from typing import Any
+
+from abogen.tts_plugin.manifest import PluginManifest
+from abogen.tts_plugin.types import EngineConfig
+
+PLUGIN_MANIFEST = PluginManifest(
+    id="invalid_capabilities",
+    name="Invalid Capabilities Plugin",
+    version="1.0.0",
+    api_version="1.0",
+    description="Plugin with unknown capabilities",
+    author="Test Author",
+    capabilities=("voice_list", "unknown_capability", "another_unknown"),
+)
+
+MODEL_REQUIREMENTS: list[Any] = []
+
+
+def create_engine(
+    context: Any,
+    model_path: Path | None,
+    config: EngineConfig,
+) -> Any:
+    raise NotImplementedError("This plugin is invalid")