SDK: sync and run import the declaring module in a child process

The command line no longer imports a research module, so import-time GPU work
ends with a short-lived child instead of holding a CUDA context for the length
of a followed run. fluksio.sdk.declare starts the child, which writes each
flow's document and shims as JSON; Declared stands in for Flow in sync().
Errors keep their type and message, a child's traceback reaches the user.

Claude-Session: https://claude.ai/code/session_01SSi5AvZ7pFmySK8zy3NPMh

backend/tests/api/routes/test_sync.py

@@ -8,7 +8,7 @@ import pytest
 from fastapi.testclient import TestClient
 
 from fluksio.core.config import settings
-from fluksio.sdk import FLOWS, MARKER, SyncError
+from fluksio.sdk import FLOWS, MARKER, Declared, SyncError
 from fluksio.sdk.client import ApiError, Client, sync
 
 EXAMPLES = Path(__file__).parents[4] / "examples"
@@ -74,6 +74,18 @@ def test_sync_stores_a_runnable_flow_with_its_origin(api, flows):
     assert "from myresearch.train import fit" in source
 
 
+def test_a_flow_read_by_another_process_syncs_as_the_flow_does(api, flows):
+    """`fluksio sync` imports in a child and uploads what it handed back."""
+    carried = Declared(flows["train"].declaration())
+
+    sync([carried], api, origin=ORIGIN)
+    stored = api.get_flow("train")["definition"]
+    assert stored["origin"]["declared_in"] == "myresearch/pipeline.py"
+
+    # Nothing differs from what the flow itself would have stored.
+    assert sync([flows["train"]], api, origin=ORIGIN)[0].unchanged
+
+
 def test_a_node_carries_the_modules_its_code_reaches(api, flows):
     """The shim names one function; the cache has to key on what it calls."""
     sync([flows["train"]], api, origin=ORIGIN)