feat: add LanceDB evaluation backend scaffold

paperforge/embedding/backends/base.py

@@ -26,4 +26,10 @@ class VectorBackend(Protocol):
 
     def delete_paper(self, paper_id: str) -> int: ...
 
+    def capabilities(self) -> dict:
+        """Return a dict describing what this backend supports.
+
+        Keys: backend, supports_hybrid, supports_multimodal.
+        """
+        ...
     def health(self) -> dict: ...

paperforge/embedding/backends/chroma_backend.py

@@ -100,3 +100,10 @@ class ChromaBackend:
                 "error": str(exc),
                 "corrupted": "hnsw" in err_lower or "corrupt" in err_lower,
             }
+
+    def capabilities(self) -> dict:
+        return {
+            "backend": "chroma",
+            "supports_hybrid": False,
+            "supports_multimodal": False,
+        }

paperforge/embedding/backends/lance_backend.py

@@ -0,0 +1,22 @@
+from __future__ import annotations
+
+import lancedb
+
+
+class LanceBackend:
+    """LanceDB-based vector backend (evaluation seam, not the default).
+
+    This backend advertises hybrid and multimodal capabilities that the
+    Chroma backend does not support, making it suitable for evaluation
+    without switching the production default.
+    """
+
+    def __init__(self, dataset_path: str) -> None:
+        self.db = lancedb.connect(str(dataset_path))
+
+    def capabilities(self) -> dict:
+        return {
+            "backend": "lancedb",
+            "supports_hybrid": True,
+            "supports_multimodal": True,
+        }

paperforge/embedding/status.py

@@ -9,6 +9,35 @@ from paperforge.embedding.backends import get_vector_backend
 
 logger = logging.getLogger(__name__)
 
+def _module_available(name: str) -> bool:
+    """Check whether *name* can be imported without side effects."""
+    import importlib
+
+    try:
+        importlib.import_module(name)
+        return True
+    except ImportError:
+        return False
+
+
+def get_available_backends() -> dict[str, dict]:
+    """Return a dict of known backends with installation and selection status."""
+    return {
+        "chroma": {
+            "installed": True,
+            "selected": True,
+            "supports_hybrid": False,
+            "supports_multimodal": False,
+        },
+        "lancedb": {
+            "installed": _module_available("lancedb"),
+            "selected": False,
+            "supports_hybrid": True,
+            "supports_multimodal": True,
+        },
+    }
+
+
 
 def get_embed_status(vault: Path) -> dict:
     """Get vector DB status. API-only mode.

tests/test_layer4_lance_backend.py

@@ -0,0 +1,44 @@
+from __future__ import annotations
+
+import pytest
+
+
+class TestLanceBackendCapabilities:
+    """LanceDB backend capability reporting (optional — skips when lancedb missing)."""
+
+    def test_lance_backend_advertises_file_based_capabilities(self):
+        pytest.importorskip("lancedb")
+        from paperforge.embedding.backends.lance_backend import LanceBackend
+
+        backend = LanceBackend("/tmp/lance")
+        caps = backend.capabilities()
+        assert caps["backend"] == "lancedb"
+        assert caps["supports_hybrid"] is True
+        assert caps["supports_multimodal"] is True
+
+
+class TestAvailableBackends:
+    """Status-level backend enumeration (always available, no lancedb dependency)."""
+
+    def test_get_available_backends_lists_chroma_and_lance(self):
+        from paperforge.embedding.status import get_available_backends
+
+        backends = get_available_backends()
+
+        assert "chroma" in backends
+        assert backends["chroma"]["installed"] is True
+        assert backends["chroma"]["selected"] is True
+
+        assert "lancedb" in backends
+        assert backends["lancedb"]["installed"] is False
+        assert backends["lancedb"]["selected"] is False
+        assert backends["lancedb"]["supports_hybrid"] is True
+        assert backends["lancedb"]["supports_multimodal"] is True
+
+    def test_get_available_backends_returns_dict_with_expected_keys(self):
+        from paperforge.embedding.status import get_available_backends
+
+        backends = get_available_backends()
+        for name, info in backends.items():
+            for key in ("installed", "selected", "supports_hybrid", "supports_multimodal"):
+                assert key in info, f"{name} missing {key}"