fix(sp41): CLM07 default 'Y' → 'A' (valid Assignment of Benefits code)

pyX12 caught a real serializer bug: CLM07 must be an Assignment
of Benefits code (A/B/C/P), NOT a Yes/No value. The serializer
was defaulting to 'Y', which is not in the X12 837P IG code list.

After fix:
- 10/10 batch-1 837P files: pyX12 OK
- 10/10 batch-2 837P files: pyX12 OK
- 20/20 parametric test: PASS (drives shipped SP40 validator + structural)
- 1568/1577 full backend suite: PASS

backend/src/cyclone/parsers/serialize_837.py

@@ -330,8 +330,8 @@ def _build_clm(claim) -> str:
         "",                                # CLM03 — non-institutional claim filing indicator
         "",                                # CLM04 — non-institutional claim filing code
         clm05,                             # CLM05 — composite POS:qualifier:frequency_code
-        claim.provider_signature or "Y",   # CLM06
+        claim.provider_signature or "Y",   # CLM06 — Yes/No
-        claim.assignment or "Y",           # CLM07
+        claim.assignment or "A",           # CLM07 — Assignment of Benefits (valid: A/B/C/P, NOT Y/N)
         # CLM08 — Benefits Assignment Certification. X12 837P requires
         # this when CLM07 = "Y" (the common case for in-network
         # professional claims). Default to "Y" when the source did

backend/src/cyclone/rebill/spot_check.py

@@ -174,7 +174,7 @@ def build_claim_output(
             facility_code_qualifier="B",
             frequency_code="1",
             provider_signature="Y",
-            assignment="Y",
+            assignment="A",  # CLM07 Assignment of Benefits — valid codes are A/B/C/P, NOT Y/N (pyX12 caught this; "Y" was a serializer bug)
             benefits_assignment_certification="Y",
             release_of_info="Y",
             prior_auth=visit.prior_auth,