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/missing_create_engine/__init__.py

@@ -1,18 +1,18 @@
-"""Invalid plugin: missing create_engine function."""
-
-from __future__ import annotations
-
-from abogen.tts_plugin.manifest import PluginManifest
-
-PLUGIN_MANIFEST = PluginManifest(
-    id="missing_create_engine",
-    name="Missing Create Engine Plugin",
-    version="1.0.0",
-    api_version="1.0",
-    description="Plugin missing create_engine",
-    author="Test Author",
-)
-
-MODEL_REQUIREMENTS: list = []
-
-# This plugin intentionally does NOT export create_engine
+"""Invalid plugin: missing create_engine function."""
+
+from __future__ import annotations
+
+from abogen.tts_plugin.manifest import PluginManifest
+
+PLUGIN_MANIFEST = PluginManifest(
+    id="missing_create_engine",
+    name="Missing Create Engine Plugin",
+    version="1.0.0",
+    api_version="1.0",
+    description="Plugin missing create_engine",
+    author="Test Author",
+)
+
+MODEL_REQUIREMENTS: list = []
+
+# This plugin intentionally does NOT export create_engine