Refuse to create a database without naming one

`create=True` had nothing to act on across a set and was accepted anyway,
leaving the first query to fail on whichever database was missing.

docs/configuration/storage.md

@@ -232,6 +232,10 @@ the same queries: retrieval MAP 0.9914 with a reranker against 0.9918 for the
 same corpus in a single database, and 0.6044 without one against 0.9798. The cost
 is that a reranker scores candidates in proportion to the number of databases.
 
+Creating names a database: `create=True` on a client covering the set raises
+`AmbiguousDatabaseError`, and `HaikuRAG(config=config, create=True,
+sources=["name"])` creates that one.
+
 Converting, chunking and title generation are functions of the configuration
 rather than of a database, so they work on a client covering the set. Writing,
 rebuilding and vacuuming name one database: asking a set-covering client raises

haiku_rag_slim/haiku/rag/client/__init__.py

@@ -230,6 +230,12 @@ class HaikuRAG:
         """
         selected = self._selected()
         if len(selected) > 1:
+            if self._create:
+                raise AmbiguousDatabaseError(
+                    "create=True creates one database, and this client covers "
+                    f"{', '.join(sorted(selected))}; name the one to create with "
+                    "sources=[name]"
+                )
             self._federated = selected
             return self
         if selected:

tests/test_multi_db.py

@@ -437,6 +437,39 @@ class TestDatabaseIndependentWork:
             assert rag.embedder is rag.store.embedder
 
 
+class TestCreatingNeedsOneDatabase:
+    """Creating names a database. Covering a set, the flag had nothing to act on
+    and was accepted anyway, leaving the first query to fail on whichever
+    database turned out to be missing."""
+
+    @pytest.mark.asyncio
+    async def test_creating_a_set_is_refused(self, tmp_path):
+        config = _config(tmp_path, ["alpha", "beta"])
+
+        with pytest.raises(AmbiguousDatabaseError, match="alpha, beta"):
+            async with HaikuRAG(config=config, create=True):
+                pass
+
+    @pytest.mark.asyncio
+    async def test_naming_one_of_the_set_creates_it(self, tmp_path):
+        config = _config(tmp_path, ["alpha", "beta"])
+
+        async with HaikuRAG(config=config, create=True, sources=["alpha"]) as rag:
+            assert await rag.count_documents() == 0
+
+        assert (tmp_path / "alpha.lancedb").exists()
+        assert not (tmp_path / "beta.lancedb").exists()
+
+    @pytest.mark.asyncio
+    async def test_covering_a_set_without_creating_is_unaffected(self, tmp_path):
+        config = _config(tmp_path, ["alpha", "beta"])
+        await _seed(config, "alpha", ["alpha one"])
+        await _seed(config, "beta", ["beta one"])
+
+        async with HaikuRAG(config=config) as rag:
+            assert await rag.count_documents() == 2
+
+
 class TestOperationsThatNeedOneDatabase:
     @pytest.mark.asyncio
     async def test_writing_names_the_databases_it_covers(self, tmp_path):