SIENTIAPDE-1975: the gateway is the only writer that updates write_status

The Laborious no longer records OPC outcomes (it inserts the row as
'pending' and only publishes the commands), so the comments and docs that
asked to keep a Laborious copy of the UPDATE in sync are updated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ingestor/io/data_sink.py

@@ -10,8 +10,8 @@ import ingestor.metrics as metrics
 _IDENTIFIER_RE = re.compile(r'\w+', re.ASCII)
 
 # Worst-wins precedence of predictions.write_status (SIENTIAPDE-1975), from best to worst;
-# NULL, 'pending' and unknown values rank 0. Same rule as the Laborious
-# (laborious/utils/write_status.py): keep both in sync.
+# NULL, 'pending' and unknown values rank 0. The gateway is the only writer that updates the
+# column: the Laborious only sets the value of the rows it inserts ('pending', PI outcome).
 STATUS_RANK = {'ok': 1, 'error': 2, 'session_bad': 3}
 
 
@@ -78,8 +78,9 @@ class DataSink:
         The row is found by timestamp + model_id. A prediction is written to several
         tags, each answered by its own command, so a better outcome never replaces a
         worse one: ``ok`` < ``error`` < ``session_bad``, and ``pending``/NULL are
-        replaced by any answer. The Laborious runs the same statement for the outcomes it
-        records itself (SIENTIAPDE-1975), so the order of the answers does not matter.
+        replaced by any answer, so the order of the answers does not matter. Only the
+        gateway updates the column (SIENTIAPDE-1975): the Laborious publishes the commands
+        and sets ``pending`` (merged with its PI Web API outcome) on the row it inserts.
 
         Never raises: a failure here must not reach the NATS callback nor hide the OPC
         write itself, which already happened. The outcome is logged and counted instead.