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/18_accountant_mixture.py

@@ -19,6 +19,14 @@ The script:
   4. For the 2-component fit derives the natural threshold (crossing of
      marginal densities in cosine-mean and dhash-mean).
 
+Firm A framing note (2026-04-20, corrected):
+  Interviews with Firm A accountants confirm MOST use replication but a
+  MINORITY may hand-sign. Firm A is thus a "replication-dominated"
+  population, NOT pure. Empirically: of ~180 Firm A accountants, ~139
+  land in C1 (high-replication) and ~32 land in C2 (middle band) under
+  the 3-component fit. The C2 Firm A members are the interview-suggested
+  minority hand-signers.
+
 Output:
   reports/accountant_mixture/accountant_mixture_report.md
   reports/accountant_mixture/accountant_mixture_results.json