fix: surface BusinessReport appendix render failures

[igerber]

Summary

• Make BusinessReport.full_report(include_appendix=True) render a visible ## Technical Appendix note ("Technical appendix unavailable: estimator summary rendering failed ()") when the estimator's summary() raises, instead of silently omitting the appendix so the report no longer looks complete when a section could not be rendered.
• Render only the exception class name — no message or traceback — to avoid leaking local paths, formulas, data values, or other internals.
• Preserve the existing code-fenced Technical Appendix output on the success path; the success and failure paths are mutually exclusive (no duplicate appendix).

Methodology references (required if estimator / math changes)

• Method name(s): N/A — reporting layer only; no estimator math, weighting, variance/SE, or inference changes
• Paper / source link(s): N/A
• Any intentional deviations from the source (and why): None

Validation

• Tests added/updated: tests/test_business_report.py — visible appendix-failure note, stringification failure, and non-leakage of raw exception details
• Backtest / simulation / notebook evidence (if applicable): N/A

Security / privacy

• Confirm no secrets/PII in this PR: Yes

🤖 Generated with Claude Code

[github-actions]

Overall Assessment

✅ Looks good

Executive Summary

• Scope is reporting-only: affected method is BusinessReport.full_report(); no estimator, weighting, variance/SE, inference, or identification logic changed.
• Methodology registry review found no affected causal method requiring paper cross-check.
• The new failure path makes appendix rendering failures visible without including exception messages or tracebacks.
• Added tests cover summary failure, stringification failure, and exception-detail non-leakage.
• I could not run pytest here because pytest is not installed; AST parsing passed for both changed files.

Methodology

No findings.

Severity: N/A
Impact: No causal estimator or statistical methodology surface is changed.
Concrete fix: None.

Code Quality

No findings.

Severity: N/A
Impact: The implementation keeps success and failure appendix paths mutually exclusive and catches both summary() and str(appendix) failures in diff_diff/business_report.py:L355-L370.
Concrete fix: None.

Performance

No findings.

Severity: N/A
Impact: The change adds only constant-time exception handling around existing appendix rendering.
Concrete fix: None.

Maintainability

No findings.

Severity: N/A
Impact: The behavior is localized to BusinessReport.full_report() with focused tests in tests/test_business_report.py:L483-L515.
Concrete fix: None.

Tech Debt

No findings.

Severity: N/A
Impact: No new TODO, deferred behavior, or tracked limitation is introduced.
Concrete fix: None.

Security

No findings.

Severity: N/A
Impact: The failure message uses only type(exc).__name__, avoiding exception messages and tracebacks that could expose local paths or sensitive values.
Concrete fix: None.

Documentation/Tests

P3 informational: Tests were not executable in this environment.

Impact: pytest is unavailable here, so I could not run tests/test_business_report.py::TestAppendix. Static AST parsing passed for both changed files.
Concrete fix: No PR change required; rely on CI to run the added appendix tests.

[igerber]

Closing unmerged — this was an internal CI-review validation run, not intended for merge.