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_manifest/__init__.py

@@ -1,10 +1,10 @@
-"""Invalid plugin: missing PLUGIN_MANIFEST."""
-
-from __future__ import annotations
-
-# This plugin intentionally does NOT export PLUGIN_MANIFEST
-MODEL_REQUIREMENTS: list = []
-
-
-def create_engine(context, model_path, config):
-    raise NotImplementedError("This plugin is invalid")
+"""Invalid plugin: missing PLUGIN_MANIFEST."""
+
+from __future__ import annotations
+
+# This plugin intentionally does NOT export PLUGIN_MANIFEST
+MODEL_REQUIREMENTS: list = []
+
+
+def create_engine(context, model_path, config):
+    raise NotImplementedError("This plugin is invalid")