Name the code a run ran, and let an interrupted sync finish

Three faults with one root: the stored body of a code-defined node is an
import shim, and nothing that mattered was ever read from the code itself.

- The run stamp could not identify what ran. The shim imports whatever is on
  disk when the worker starts, and an uncommitted tree stamps <commit>-dirty
  for every run it ever produces. Run.code_digest hashes the repository's .py
  files, memoized on their stat state, and it is read again when the run is
  actually claimed -- so a sweep queued for hours records the code each of its
  runs executed, not the code that was there when it was submitted.
- The stage cache adopted code that was too new. The fingerprint hashed the
  shim, which is invariant under any edit to the imported function or anything
  it calls into, so a re-run was served from cache and answered without the
  outputs the edit added. It now carries the repo digest and the node's
  declared ports. Every fingerprint changes once, which invalidates the
  existing cache; a canvas flow has no repository and keys as before.
- An interrupted sync looked like a hand-edited canvas. The engine answers a
  new-node template for a node with no stored body, and the template carries
  no marker, so the drift check read "somebody edited this" and demanded
  --force -- for the one state that re-running the sync is the fix for.
  NodeSource.missing states the fact, and sync skips those and reuses the
  bodies it read instead of asking for each one twice.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/fluksio/flow/store.py

@@ -549,6 +549,18 @@ class FlowStore:
         path = self._node_file(flow, node_id)
         return path.read_text() if path.exists() else DEFAULT_SOURCE
 
+    def has_node_source(self, flow: str, node_id: str, draft: bool = False) -> bool:
+        """Whether a body was ever stored for this node.
+
+        `read_node_source` answers a template when there is none, which is what
+        a new node's editor should open with — but it is a placeholder, not
+        code anybody wrote, and telling the two apart is the difference between
+        "someone edited this" and "this was never written".
+        """
+        if draft and self._draft_node_file(flow, node_id).exists():
+            return True
+        return self._node_file(flow, node_id).exists()
+
     def write_node_source(
         self, flow: str, node_id: str, code: str, draft: bool = False
     ) -> bool: