feat(sp24): expose PATIENT_LOOP_DEFAULT_INCLUDED constant in serialize_837

Promotes the default value of _build_subscriber_block(include_patient_loop=...)
from a hardcoded True (the broken IG-incorrect state that emitted HL*3 /
PAT*01 / NM1*QC for SBR02='18' claims) to a public module-level constant
PATIENT_LOOP_DEFAULT_INCLUDED = False (the IG-correct state).

Per X12 005010X222A1, the 2000C Patient Hierarchical Level MUST be absent
when SBR02 == '18' (Self-pay, the CO-Medicaid IHSS workflow). The previous
default of True emitted the forbidden 2000C block on every self-pay claim,
which Edifabric rejected with '2000C HL must be absent when 2000B SBR02 = "18"'.

Also adds __all__ to the module so PATIENT_LOOP_DEFAULT_INCLUDED is
documented as part of the public surface. The IG-correctness guard
(see cyclone.reissue.ig_correctness_check in the next commit) reads this
constant rather than inspect.signature'ing _build_subscriber_block —
that handle survives any future refactor of the helper.

backend/src/cyclone/parsers/serialize_837.py

@@ -52,6 +52,14 @@ from types import SimpleNamespace
 
 from cyclone.parsers.models import ClaimOutput
 
+__all__ = [
+    "PATIENT_LOOP_DEFAULT_INCLUDED",
+    "SerializeError",
+    "serialize_837",
+    "serialize_837_for_resubmit",
+    "serialize_member_week_batch",
+]
+
 _SEG = "~"
 _ELEM = "*"
 _ISA_COMPONENT_SEPARATOR = ":"
@@ -64,6 +72,22 @@ class SerializeError(Exception):
     """Raised when a claim cannot be serialized."""
 
 
+#: Default value for ``_build_subscriber_block(include_patient_loop=...)``.
+#:
+#: 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 regression test
+#: ``tests/test_serialize_837.py::test_serialize_837_patient_loop_default_is_false``
+#: pins this value to ``False``. Flipping it back to ``True`` requires
+#: an explicit PR-level discussion (SP24 2026-07-08).
+PATIENT_LOOP_DEFAULT_INCLUDED: bool = False
+
+
 # ---------------------------------------------------------------------------
 # Envelope helpers
 # ---------------------------------------------------------------------------
@@ -502,7 +526,7 @@ def _build_subscriber_block(
     subscriber,
     claim_filing_indicator_code: str | None,
     payer=None,
-    include_patient_loop: bool = True,
+    include_patient_loop: bool = PATIENT_LOOP_DEFAULT_INCLUDED,
 ) -> list[str]:
     """Loop 2000B (HL*2) → SBR → 2010BA (NM1*IL) → 2010BB (NM1*PR).