Fix stacked DiD tutorial: remove warning from trimming fix cell

[igerber]

Summary

• Remove redundant StackedDiD(kappa_pre=3, kappa_post=3) call from the trimming fix cell in tutorial 13
• The symmetric-kappa case re-emitted the trimming warning that was already shown and explained in preceding cells, undermining the narrative of "here's how to fix it"
• The cell now only shows the asymmetric window solution, which genuinely runs warning-free

Methodology references (required if estimator / math changes)

• N/A - no methodology changes (tutorial-only fix)

Validation

• Tests added/updated: No test changes (tutorial narrative fix)
• Backtest / simulation / notebook evidence: Tutorial cell output will no longer show a spurious warning

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

• Simplifies the trimming-fix example by removing the redundant symmetric kappa run; asymmetric window example remains in docs/tutorials/13_stacked_did.ipynb:L242-L247.
• No estimator, math, variance/SE, or assumption logic changes; StackedDiD methodology registry entry remains the reference (docs/methodology/REGISTRY.md:L643-L720).
• Edge-case checklist and inference integrity are unaffected because no code paths changed; tests unchanged (reasonable for tutorial-only update).

Methodology
No findings. Tutorial-only edit; no estimator changes relative to the StackedDiD registry entry (docs/methodology/REGISTRY.md:L643-L720).

Code Quality
No findings.

Performance
No findings.

Maintainability
No findings.

Tech Debt
No findings.

Security
No findings.

Documentation/Tests
No findings. Tutorial-only update; no tests expected.