feat: add plugin loader infrastructure

Implement plugin loading and validation:
- loader.py: discover, import, validate plugins
- validate PLUGIN_MANIFEST, MODEL_REQUIREMENTS, create_engine
- validate api_version compatibility (major must match)
- validate capabilities (reject unknown)
- diagnostic messages for all error cases
- no partial registration after error

Test plugins:
- fake_plugin: minimal valid plugin for testing
- missing_manifest: no PLUGIN_MANIFEST
- invalid_api_version: major version mismatch
- invalid_capabilities: unknown capabilities
- missing_create_engine: no create_engine function
- import_error: raises ImportError during import
- missing_model_requirements: no MODEL_REQUIREMENTS

39 new tests covering all loader functionality.

tests/plugins/fake_plugin/__init__.py

@@ -0,0 +1,92 @@
+"""Fake plugin for testing the plugin loader.
+
+This is a minimal valid plugin that satisfies the Plugin API contract.
+It does NOT perform any real TTS synthesis.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+from typing import Any
+
+from abogen.tts_plugin.engine import Engine, EngineSession
+from abogen.tts_plugin.errors import EngineError
+from abogen.tts_plugin.host_context import HostContext
+from abogen.tts_plugin.manifest import (
+    AudioFormatManifest,
+    EngineManifest,
+    PluginManifest,
+    VoiceSourceManifest,
+)
+from abogen.tts_plugin.types import (
+    AudioFormat,
+    Duration,
+    EngineConfig,
+    ParameterValues,
+    SynthesisRequest,
+    SynthesizedAudio,
+)
+
+
+class FakeSession:
+    """Minimal EngineSession implementation for testing."""
+
+    def __init__(self) -> None:
+        self._disposed = False
+
+    def synthesize(self, request: SynthesisRequest) -> SynthesizedAudio:
+        if self._disposed:
+            raise EngineError("Session disposed")
+        return SynthesizedAudio(
+            data=b"\x00" * 100,
+            format=AudioFormat(mime="audio/wav", extension="wav"),
+            duration=Duration(seconds=1.0),
+        )
+
+    def dispose(self) -> None:
+        self._disposed = True
+
+
+class FakeEngine:
+    """Minimal Engine implementation for testing."""
+
+    def __init__(self) -> None:
+        self._disposed = False
+
+    def createSession(self) -> EngineSession:
+        if self._disposed:
+            raise EngineError("Engine disposed")
+        return FakeSession()
+
+    def dispose(self) -> None:
+        self._disposed = True
+
+
+PLUGIN_MANIFEST = PluginManifest(
+    id="fake_plugin",
+    name="Fake Plugin",
+    version="1.0.0",
+    api_version="1.0",
+    description="A fake plugin for testing",
+    author="Test Author",
+    capabilities=(),
+    engine=EngineManifest(
+        voiceSources=(
+            VoiceSourceManifest(id="builtin", name="Builtin", type="list"),
+        ),
+        audioFormats=(
+            AudioFormatManifest(mime="audio/wav", extension="wav"),
+        ),
+    ),
+)
+
+MODEL_REQUIREMENTS: list[Any] = []
+
+
+def create_engine(
+    context: HostContext,
+    model_path: Path | None,
+    config: EngineConfig,
+) -> Engine:
+    """Create a fake engine instance."""
+    return FakeEngine()

tests/plugins/import_error/__init__.py

@@ -0,0 +1,18 @@
+"""Invalid plugin: raises ImportError during import."""
+
+from __future__ import annotations
+
+# This plugin intentionally raises an ImportError
+raise ImportError("Simulated import error for testing")
+
+# The following code will never be reached, but is here for documentation
+from abogen.tts_plugin.manifest import PluginManifest
+
+PLUGIN_MANIFEST = PluginManifest(
+    id="import_error",
+    name="Import Error Plugin",
+    version="1.0.0",
+    api_version="1.0",
+    description="Plugin that fails to import",
+    author="Test Author",
+)

tests/plugins/invalid_api_version/__init__.py

@@ -0,0 +1,29 @@
+"""Invalid plugin: incompatible api_version (major version mismatch)."""
+
+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
+
+# api_version "2.0" has major version 2, but host expects major version 1
+PLUGIN_MANIFEST = PluginManifest(
+    id="invalid_api_version",
+    name="Invalid API Version Plugin",
+    version="1.0.0",
+    api_version="2.0",  # Major version mismatch!
+    description="Plugin with incompatible api_version",
+    author="Test Author",
+)
+
+MODEL_REQUIREMENTS: list[Any] = []
+
+
+def create_engine(
+    context: Any,
+    model_path: Path | None,
+    config: EngineConfig,
+) -> Any:
+    raise NotImplementedError("This plugin is invalid")

tests/plugins/invalid_capabilities/__init__.py

@@ -0,0 +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")

tests/plugins/missing_create_engine/__init__.py

@@ -0,0 +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

tests/plugins/missing_manifest/__init__.py

@@ -0,0 +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")

tests/plugins/missing_model_requirements/__init__.py

@@ -0,0 +1,28 @@
+"""Invalid plugin: missing MODEL_REQUIREMENTS."""
+
+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="missing_model_requirements",
+    name="Missing Model Requirements Plugin",
+    version="1.0.0",
+    api_version="1.0",
+    description="Plugin missing MODEL_REQUIREMENTS",
+    author="Test Author",
+)
+
+# This plugin intentionally does NOT export MODEL_REQUIREMENTS
+
+
+def create_engine(
+    context: Any,
+    model_path: Path | None,
+    config: EngineConfig,
+) -> Any:
+    raise NotImplementedError("This plugin is invalid")