Export FLOWS and build from the top-level fluksio package

A test of the declarations had to import them from fluksio.sdk while the
guide teaches `from fluksio import ...`. The worker's inert module gets the
same two names so a module importing them still loads inside a node.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QgG6TJw5qBZzaLghQSRVrq

backend/tests/sdk/test_build.py

@@ -364,3 +364,15 @@ def test_a_node_in_a_script_run_directly_is_refused_at_sync_not_at_import():
         flow.document()
     with pytest.raises(SyncError, match="run directly"):
         flow.shims()
+
+
+def test_the_registry_and_the_builder_come_with_the_authoring_import():
+    """A test of the declarations needs no second import path."""
+    import fluksio
+    import fluksio.sdk
+
+    flow = a_flow()
+
+    assert fluksio.FLOWS is fluksio.sdk.FLOWS
+    assert fluksio.FLOWS["train"] is flow
+    assert fluksio.build(flow) == fluksio.sdk.build(flow)