Update methodology review for MultiPeriodDiD

[igerber]

Summary

• Mark MultiPeriodDiD as Complete in METHODOLOGY_REVIEW.md (was "Not Started")
• Add 16 verified components, test coverage, corrections, and outstanding concerns
• Add 6 missing edge cases to REGISTRY.md (post-period reference validation, FutureWarning, NaN inference, treatment reversal/D_it warnings)

PR #125 did extensive methodology work on MultiPeriodDiD (7 review rounds, full event-study overhaul) but METHODOLOGY_REVIEW.md was not updated to reflect this.

Methodology references (required if estimator / math changes)

• Method name(s): N/A - documentation updates only, no estimator code changes
• Paper / source link(s): N/A
• Any intentional deviations from the source (and why): None

Validation

• Tests added/updated: No test changes (documentation only)
• Backtest / simulation / notebook evidence (if applicable): N/A

Security / privacy

• Confirm no secrets/PII in this PR: Yes

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Generated with Claude Code

[github-actions]

Overall assessment: ✅ Looks good

Executive summary

• Documentation-only update marking MultiPeriodDiD as complete and expanding registry edge cases; no estimator code changes in this PR (METHODOLOGY_REVIEW.md:19).
• Registry additions (post-period reference error, FutureWarning, NaN inference, treatment reversal/D_it warnings) are consistent with existing MultiPeriodDiD behavior and docstrings (docs/methodology/REGISTRY.md:160, diff_diff/estimators.py:651).
• No new tests or benchmarks added; changes are descriptive only.

Methodology

• No findings.

Code Quality

• No findings.

Performance

• No findings.

Maintainability

• No findings.

Tech Debt

• No findings.

Security

• No findings.

Documentation/Tests

• No findings.