feat: add frozen Plugin API skeleton

Create public API structure for TTS Plugin Architecture:
- types.py: immutable value objects (AudioFormat, Duration, VoiceSelection, etc.)
- errors.py: EngineError hierarchy (7 typed exceptions)
- manifest.py: plugin manifest dataclasses (PluginManifest, EngineManifest, etc.)
- engine.py: Engine and EngineSession protocols
- capabilities.py: optional capability interfaces (VoiceLister, PreviewGenerator, etc.)
- host_context.py: HostContext and HttpClient protocol
- plugin.py: plugin contract (create_engine signature)
- __init__.py: public API exports

All interfaces are fully defined but contain no business logic.
API is frozen and ready for implementation in subsequent PRs.

abogen/tts_plugin/host_context.py

@@ -0,0 +1,46 @@
+"""Host context for the TTS Plugin Architecture.
+
+This module defines the HostContext dataclass that provides minimal
+host services to plugins. It is the only interface through which
+plugins can access host functionality.
+"""
+
+from __future__ import annotations
+
+import logging
+from dataclasses import dataclass
+from pathlib import Path
+from typing import Protocol, runtime_checkable
+
+
+@runtime_checkable
+class HttpClient(Protocol):
+    """Protocol for HTTP client provided by host.
+
+    Plugins can use this for network requests (e.g., API-based engines).
+    """
+
+    def get(self, url: str, **kwargs: object) -> object:
+        """Perform an HTTP GET request."""
+        ...
+
+    def post(self, url: str, **kwargs: object) -> object:
+        """Perform an HTTP POST request."""
+        ...
+
+
+@dataclass(frozen=True)
+class HostContext:
+    """Minimal host context provided to plugins.
+
+    Contains only essential host services. No business logic.
+
+    Attributes:
+        config_dir: Directory for API keys, preferences, and configuration.
+        logger: Logger for plugin logging.
+        http_client: HTTP client for network requests.
+    """
+
+    config_dir: Path
+    logger: logging.Logger
+    http_client: HttpClient