Add matplotlib import guards to tutorials 11 and 12

[igerber]

Summary

• Add try/except ImportError matplotlib guard to cell 1 of tutorials 11 (Imputation DiD) and 12 (Two-Stage DiD)
• Wrap plot_event_study() calls in cell 5 of both tutorials with if HAS_MATPLOTLIB: guard
• Matches the pattern used by all other tutorials (02, 03, 05, etc.)

Methodology references (required if estimator / math changes)

• N/A - no methodology changes

Validation

• Tests added/updated: No test changes (tutorial-only fix)
• Backtest / simulation / notebook evidence: Both notebooks validated via jupyter nbconvert --execute — all cells pass

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

• Added optional matplotlib import/style and plot guards in tutorials 11 and 12, making visualization cells skippable when matplotlib isn’t installed. docs/tutorials/11_imputation_did.ipynb:L22-L93, docs/tutorials/12_two_stage_did.ipynb:L22-L95
• No estimator/math/inference changes; tutorial methodology statements for ImputationDiD and TwoStageDiD remain consistent with the Methodology Registry and estimator docstrings. docs/tutorials/11_imputation_did.ipynb:L6-L14, docs/tutorials/12_two_stage_did.ipynb:L141-L147, docs/methodology/REGISTRY.md:L464-L638, diff_diff/imputation.py:L447-L462, diff_diff/two_stage.py:L407-L420
• Minor robustness suggestion for matplotlib style handling (see Code Quality).

Methodology

• P3 (Info) — Impact: None; Concrete fix: None (tutorial statements remain aligned with the Methodology Registry and estimator docstrings). docs/tutorials/11_imputation_did.ipynb:L6-L14, docs/tutorials/12_two_stage_did.ipynb:L141-L147, docs/methodology/REGISTRY.md:L464-L638, diff_diff/imputation.py:L447-L462, diff_diff/two_stage.py:L407-L420

Code Quality

• P3 — Impact: If matplotlib is installed but the seaborn-v0_8-whitegrid style is unavailable (older matplotlib), the tutorial will error despite the optional-plot guard. Concrete fix: wrap plt.style.use(...) in a broader try/except (e.g., catch OSError) and fall back to default style while keeping HAS_MATPLOTLIB=True. docs/tutorials/11_imputation_did.ipynb:L32-L39, docs/tutorials/12_two_stage_did.ipynb:L32-L39

Performance

• P3 (Info) — Impact: None; Concrete fix: None (docs-only change).

Maintainability

• P3 (Info) — Impact: None; Concrete fix: None (docs-only change).

Tech Debt

• P3 (Info) — Impact: None; Concrete fix: None.

Security

• P3 (Info) — Impact: None; Concrete fix: None (no new I/O or secrets).

Documentation/Tests

• P3 (Info) — Impact: None; Concrete fix: None (tutorial-only update; no tests expected).