test(sp24): regression test pinning PATIENT_LOOP_DEFAULT_INCLUDED to False

Adds test_serialize_837_patient_loop_default_is_false — the long-term
guard against the SP24 bug coming back via a future refactor:

  1. Asserts the module-level PATIENT_LOOP_DEFAULT_INCLUDED constant
     is exactly False (not just falsy).

  2. Calls _build_subscriber_block with no kwarg and verifies the
     output contains no HL*3 segment + HL*2 child_count = 0.

  3. The assertion message points at the SP24 spec §2 Decision 2 so
     a future maintainer sees the regression story immediately.

Autoreview: confirmed the test fails when the constant is flipped
to True (the broken pattern from 2026-07-08), with the expected
'PATIENT_LOOP_DEFAULT_INCLUDED must be False' assertion message.

backend/tests/test_serialize_837.py

@@ -738,3 +738,69 @@ def test_sbr_segment_respects_explicit_claim_filing_indicator():
     sbr_line = next(s for s in text.split("~") if s.startswith("SBR"))
     parts = sbr_line.split("*")
     assert parts[9] == "16", f"SBR-09 should reflect explicit kwarg, got {parts[9]!r}"
+
+
+# ---------------------------------------------------------------------------
+# SP24 — IG-correctness regression test
+#
+# The 2000C Patient Hierarchical Level (HL*3 → PAT → NM1*QC) MUST be
+# absent when SBR02 == "18" (Self-pay, the CO-Medicaid IHSS workflow).
+# This test pins the PATIENT_LOOP_DEFAULT_INCLUDED constant to False so
+# a future refactor cannot silently reintroduce the broken pattern that
+# surfaced in the 2026-07-08 Edifabric 999 audit.
+# ---------------------------------------------------------------------------
+
+
+def test_serialize_837_patient_loop_default_is_false():
+    """SP24 regression: PATIENT_LOOP_DEFAULT_INCLUDED must stay False.
+
+    Per X12 005010X222A1, the 2000C Patient Hierarchical Level
+    (``HL*3 → PAT → NM1*QC``) is REQUIRED only when Patient != Subscriber
+    (i.e. ``SBR02 != "18"``). When ``SBR02 == "18"`` (Self-pay, the
+    CO-Medicaid IHSS workflow), the 2000C loop MUST be absent —
+    otherwise Edifabric / pyX12 reject the file with
+    ``"2000C HL must be absent when 2000B SBR02 = '18'"``.
+
+    The serializer encodes this invariant as a module-level constant
+    ``PATIENT_LOOP_DEFAULT_INCLUDED``. Flipping the default back to
+    ``True`` would re-introduce the rejection on every self-pay claim
+    in the dzinesco pipeline. This test pins the constant + verifies
+    the helper actually honors it on a no-kwarg call.
+
+    Reference: ``docs/superpowers/specs/2026-07-08-cyclone-reissue-claims-design.md``
+    """
+    from datetime import date as _date
+    from cyclone.parsers.models import Subscriber
+    from cyclone.parsers.serialize_837 import (
+        PATIENT_LOOP_DEFAULT_INCLUDED,
+        _build_subscriber_block,
+    )
+
+    # 1. The constant itself is False — the IG-correct shape.
+    assert PATIENT_LOOP_DEFAULT_INCLUDED is False, (
+        f"PATIENT_LOOP_DEFAULT_INCLUDED must be False "
+        f"(got {PATIENT_LOOP_DEFAULT_INCLUDED!r}); the IG-correct "
+        f"serializer shape for SBR02='18' claims is 2000C-absent. "
+        f"See the SP24 spec §2 Decision 2 for the regression story."
+    )
+
+    # 2. Calling _build_subscriber_block with no kwarg honors the
+    #    constant — no HL*3 segment emitted, HL*2 child count = 0.
+    sub = Subscriber(
+        member_id="TEST123",
+        first_name="Test",
+        last_name="Patient",
+        dob=_date(1990, 1, 1),
+        gender="F",
+    )
+    out = _build_subscriber_block(sub, "MC")
+    assert not any(seg.startswith("HL*3") for seg in out), (
+        f"HL*3 emitted with the IG-correct default; segments={out!r}"
+    )
+    # HL*2 child_count must be 0, not 1.
+    hl2_line = next(seg for seg in out if seg.startswith("HL*2"))
+    hl2_parts = hl2_line.rstrip("~").split("*")
+    assert hl2_parts[4] == "0", (
+        f"HL*2 child_count must be 0 (no 2000C children); got {hl2_parts[4]!r}. "
+        f"Line: {hl2_line!r}"
+    )