Correct Firm A framing: replication-dominated, not pure

Interview evidence from multiple Firm A accountants confirms that MOST
use replication (stamping / firm-level e-signing) but a MINORITY may
still hand-sign. Firm A is therefore a "replication-dominated" population,
not a "pure" one. This framing is consistent with:

- 92.5% of Firm A signatures exceed cosine 0.95 (majority replication)
- The long left tail (~7%) captures the minority hand-signers, not scan
  noise or preprocessing artifacts
- Hartigan dip test: Firm A cosine unimodal long-tail (p=0.17)
- Accountant-level GMM: of 180 Firm A accountants, 139 cluster in C1
  (high-replication) and 32 in C2 (middle band = minority hand-signers)

Updates docstrings and report text in Scripts 15, 16, 18, 19 to match.
Partner v3's "near-universal non-hand-signing" language corrected.

Script 19 regenerated with the updated text.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

signature_analysis/16_bd_mccrary_discontinuity.py

@@ -306,8 +306,10 @@ def main():
         '* Multiple candidate transitions are ranked by total |Z| magnitude',
         '  on both sides of the boundary; the strongest is reported.',
         '* Absence of a significant transition is itself informative: it',
-        '  is consistent with a single generative mechanism (e.g. Firm A',
-        '  which is near-universally non-hand-signed).',
+        '  is consistent with a single dominant generative mechanism (e.g.',
+        '  Firm A, a replication-dominated population per interviews with',
+        '  multiple Firm A accountants -- most use replication, a minority',
+        '  may hand-sign).',
     ]
     md_path = OUT / 'bd_mccrary_report.md'
     md_path.write_text('\n'.join(md), encoding='utf-8')