fix: tolerate spreadsheet decimal roundoff

.project-docs/50-evidence/topics/2026-08-04-report-artifact-deployability.md

@@ -27,6 +27,20 @@
 - The Docker root dependency chain now installs `openpyxl==3.1.5` through `requirements-monthly-reports.txt`; no
   separate npm or Codex-runtime bootstrap is needed.
 
+## Follow-up: 2026-08-03 monthly validation failure
+
+- Operator-provided production evidence showed a 440-row, multi-sheet 2026-08-03 workbook whose sheet names and
+  channel counts matched, and which passed manual validation when reopened non-read-only with numeric tolerance. The
+  worker nevertheless emitted `MONTHLY_REPORT_OUTPUT_VALIDATION_FAILED`; outbox event 31 became `dead` and event 30
+  remained pending/retrying. This isolates the failure to the workbook-validation implementation rather than
+  container, worker startup, database or OSS publication.
+- `_validate_workbook` no longer calls `worksheet.cell(row, column)` on a `read_only=True` workbook. It consumes each
+  sheet header and data row in order with `iter_rows`, rejects missing/extra rows or columns, and keeps strict formula
+  and semantic checks. Excel-decimal values now use a finite-Decimal absolute tolerance of `0.000001`.
+- Regression coverage builds 440 rows across all five standard sheets over three daily versions, with decimal rate and
+  price values plus KB rows. The builder validates 440 formulas successfully; the dedicated XLSX class passes 2/2 and
+  the monthly builder/publishing/worker/service/repository slice passes 18/18.
+
 ## Verification
 
 - Targeted report/web/storage/migration suite: 41 tests passed; builder/company integration slice: 15 tests passed