fix(sp37): sync head-assertion + update batch_envelope_index docstrings

- test_acks.py:test_migration_latest_idempotent_on_fresh_db was still
  asserting user_version == 19; migration 0020 (Task 1) bumped it to 20.
  Without this fix, the full pytest suite fails the moment SP37 ships.
- claim_acks.py module docstring + store/__init__.py facade docstring
  still advertised the index as single-key {envelope.control_number:
  batch.id}. SP37 Task 3 made it dual-key (ISA13 + ST02); the docs now
  match the implementation.

backend/src/cyclone/store/__init__.py

@@ -325,7 +325,13 @@ class CycloneStore:
         return _remove_claim_ack(link_id, event_bus=event_bus)
 
     def batch_envelope_index(self):
-        """Return a {envelope.control_number: batch.id} map for D10 Pass 1."""
+        """Return a {key: batch.id} map populated from two columns (D10 Pass 1).
+
+        Each 837p batch contributes both ``Batch.raw_result_json.envelope.control_number``
+        (ISA13) and ``Batch.transaction_set_control_number`` (ST02); 835 batches
+        are excluded. Single ``.get(set_control_number)`` lookup resolves
+        either key — SP37 closes the 999 AK201 → source-batch gap.
+        """
         return _batch_envelope_index()
 
     # -- SP17: encrypted DB backups -------------------------------------

backend/src/cyclone/store/claim_acks.py

@@ -11,8 +11,10 @@ Five methods on top of the new ``ClaimAck`` ORM table:
   ack-orphans lane).
 * ``remove_claim_ack`` — unlink. Publishes ``claim_ack_dropped``.
 * ``batch_envelope_index`` — D10 in-memory map of
-  ``Batch.envelope.control_number → batch.id`` (cheap to rebuild;
-  re-built once per ingest).
+  ``{Batch.raw_result_json.envelope.control_number (ISA13) OR
+  Batch.transaction_set_control_number (ST02)} → batch.id``
+  (SP37: populated from two columns; cheap to rebuild, re-built
+  once per ingest).
 
 Each mutating method opens its own ``db.SessionLocal()()`` session
 so callers don't have to manage session lifecycles. Publishes are

backend/tests/test_acks.py

@@ -51,7 +51,7 @@ def test_migration_0002_creates_acks_table():
 
 def test_migration_latest_idempotent_on_fresh_db():
     """Re-running the migration on the same DB must be a no-op (PRAGMA
-    user_version already at the latest version — currently 19 after
+    user_version already at the latest version — currently 20 after
     0004-0006 line_reconciliation, 0005 ta1_acks, SP9's 0007
     providers/payers/clearhouse, SP10's 0008 payer_rejected,
     SP11's 0009 audit_log, SP14's 0010 payer_rejected_acknowledged,
@@ -61,15 +61,16 @@ def test_migration_latest_idempotent_on_fresh_db():
     claims.matched_remittance_id index, SP27-Task 17's 0017
     claim.patient_control_number backfill UPDATE, SP28's 0018
     claim_acks join table, SP32's 0019
-    rendering_provider_npi + service_provider_npi)."""
+    rendering_provider_npi + service_provider_npi, and SP37's 0020
+    transaction_set_control_number)."""
     with db.engine().begin() as c:
         v1 = c.exec_driver_sql("PRAGMA user_version").scalar() or 0
-    assert v1 == 19
+    assert v1 == 20
     # A second run should not raise and should not bump the version.
     db_migrate.run(db.engine())
     with db.engine().begin() as c:
         v2 = c.exec_driver_sql("PRAGMA user_version").scalar() or 0
-    assert v2 == 19
+    assert v2 == 20
 
 
 def test_add_ack_persists_row():