feat(llm): add embedding adapter with cache and similarity utils (S1.3)

Add OpenAI-compatible embedding support (works with both OpenAI and
OpenRouter), file-based embedding cache with content-digest invalidation,
and pure-Python cosine similarity utilities for downstream redundancy
detection.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

markitect/llm/embedding_factory.py

@@ -0,0 +1,50 @@
+"""
+Factory for creating embedding adapters by provider name.
+"""
+
+from typing import Optional, Any
+
+from markitect.llm.embedding_adapter import EmbeddingAdapter
+from markitect.llm.exceptions import LLMConfigurationError
+
+_EMBEDDING_PROVIDERS = {
+    "openai": "markitect.llm.embedding_openai.OpenAICompatibleEmbeddingAdapter",
+    "openrouter": "markitect.llm.embedding_openai.OpenAICompatibleEmbeddingAdapter",
+}
+
+
+def create_embedding_adapter(
+    provider: str = "openai",
+    model: Optional[str] = None,
+    api_key: Optional[str] = None,
+    **kwargs: Any,
+) -> EmbeddingAdapter:
+    """Instantiate an :class:`EmbeddingAdapter` for the given *provider*.
+
+    Args:
+        provider: ``"openai"`` or ``"openrouter"``.
+        model: Embedding model name (e.g. ``"text-embedding-3-small"``).
+        api_key: Explicit API key.
+        **kwargs: Extra keyword arguments forwarded to the adapter.
+
+    Returns:
+        A ready-to-use :class:`EmbeddingAdapter` instance.
+
+    Raises:
+        LLMConfigurationError: If *provider* is not recognised.
+    """
+    if provider not in _EMBEDDING_PROVIDERS:
+        known = ", ".join(sorted(_EMBEDDING_PROVIDERS))
+        raise LLMConfigurationError(
+            f"Unknown embedding provider {provider!r}. Choose from: {known}",
+            context={"provider": provider},
+        )
+
+    # Lazy import
+    fqn = _EMBEDDING_PROVIDERS[provider]
+    module_path, class_name = fqn.rsplit(".", 1)
+    import importlib
+    mod = importlib.import_module(module_path)
+    cls = getattr(mod, class_name)
+
+    return cls(model=model, api_key=api_key, provider=provider, **kwargs)