feat: add static voice catalog to PluginManifest

- Add  to PluginManifest
  - None = not declared (use VoiceLister fallback)
  - () = explicitly no static voices
  - Non-empty = static catalog available without Engine instantiation

- Update get_voices() to check manifest first, fall back to Engine
- Declare 54 Kokoro voices and 10 SuperTonic voices in manifests
- Remove hardcoded voice lists from engine.py files
- Engine.listVoices() now returns [] (manifest is source of truth)

- Clean up dead create_pipeline() kwargs (sample_rate, auto_download, total_steps)
  - SuperTonic plugin uses internal defaults
  - total_steps is per-request parameter via Pipeline.__call__() kwargs

- Add clear_preview_pipelines() for resource cleanup
- Fix test mocks to override listVoices()
- Update Architecture Amendment #1 doc

docs/architecture-amendment-001.md

@@ -0,0 +1,89 @@
+# Architecture Amendment #1: EngineConfig — `lang_code` field
+
+**Date:** 2026-07-12
+**Status:** Accepted
+**PR:** #12 (Normalize Pipeline Public API)
+
+## Summary
+
+Add `lang_code: str = "a"` to `EngineConfig` and update its definition to clarify the architectural contract.
+
+## Background
+
+During migration from the old `KokoroBackend` to the Plugin Architecture, the `lang_code` parameter became a dead argument. The old backend read it from `**kwargs` and passed it to `KPipeline(lang_code=...)`. The new `KokoroPlugin.create_engine()` hardcodes `lang_code="a"`, ignoring the config entirely. Callers continued passing `lang_code` to `create_pipeline()`, unaware it had no effect.
+
+This is a functional regression relative to the pre-Plugin Architecture behavior.
+
+## Decision
+
+### 1. Updated EngineConfig definition
+
+**Before:**
+```
+Immutable value object for engine initialization settings.
+Contains only engine-specific settings, no resource references.
+```
+
+**After:**
+```
+Immutable configuration of an Engine instance.
+Contains parameters that define how a particular Engine instance is
+created and that remain constant throughout the lifetime of that Engine.
+Plugin implementations may ignore fields that are not applicable to them.
+```
+
+### 2. New field
+
+```python
+@dataclass(frozen=True)
+class EngineConfig:
+    device: str = "cpu"
+    lang_code: str = "a"
+```
+
+### 3. Architectural rules
+
+- **Fields in EngineConfig are optional unless explicitly required by a plugin.**
+- **Plugins MUST ignore unsupported EngineConfig fields.**
+- **All parameters that may vary between individual synthesis requests must remain in `SynthesisRequest.parameters`.**
+
+## Rationale
+
+Analysis of real TTS engines (Kokoro, Piper, XTTS, Coqui, StyleTTS2, Fish Speech) confirmed:
+
+| Parameter type | Where it belongs | Example |
+|---------------|-----------------|---------|
+| Engine instance config (immutable) | `EngineConfig` | `device`, `lang_code` |
+| Synthesis parameters (per-request) | `SynthesisRequest.parameters` | `speed`, `split_pattern`, `total_steps` |
+
+`lang_code` determines the engine's behavior at creation time and cannot be changed during the engine's lifetime. It is not a synthesis parameter.
+
+## Impact on existing plugins
+
+| Plugin | `device` | `lang_code` | Notes |
+|--------|----------|-------------|-------|
+| Kokoro | Reads ✓ | Reads ✓ (was hardcoded, now from config) | Regression fixed |
+| SuperTonic | Ignores | Ignores | No change — no language concept |
+| Future plugins | May read | May ignore | Field-ignoring rule applies |
+
+## Contract tests added
+
+```python
+class TestEngineConfigContract:
+    def test_default_lang_code(self)       # EngineConfig().lang_code == "a"
+    def test_custom_lang_code(self)        # EngineConfig(lang_code="j").lang_code == "j"
+    def test_immutability_lang_code(self)  # frozen — cannot reassign
+    def test_plugins_may_ignore_irrelevant_fields(self)  # field-ignoring rule
+    def test_engine_config_contains_engine_instance_configuration(self)  # definition
+```
+
+## Files changed
+
+| File | Change |
+|------|--------|
+| `abogen/tts_plugin/types.py` | Updated docstring, added `lang_code: str = "a"` |
+| `plugins/kokoro/__init__.py` | Reads `config.lang_code` instead of hardcoded `"a"` |
+| `abogen/tts_plugin/utils.py` | `create_pipeline()` passes `lang_code` to `EngineConfig` |
+| `tests/contracts/test_types_contract.py` | 5 new contract tests |
+| `tests/contracts/test_plugin_manager_contract.py` | Updated assertion for `lang_code` |
+| `tests/test_behavioral_regression.py` | Updated `test_engine_config_defaults` |