docs: replace README hero image with corrected DiD diagram

[igerber]

Summary

• Replace diff-diff.png (the README hero image) with a corrected version - the old diagram was inaccurate
• New image shows a correct DiD illustration: parallel pre-trends, the treated counterfactual continuing the control trend after the intervention, and the DiD effect as the treated-vs-counterfactual gap
• Same dimensions as before (~1730x910), so the README layout and the raw.githubusercontent URL reference are unchanged (no README.md edit needed)

Methodology references (required if estimator / math changes)

• Method name(s): N/A - no methodology changes (image asset only)
• Paper / source link(s): N/A
• Any intentional deviations from the source (and why): None

Validation

• Tests added/updated: No test changes (binary image asset only)
• Backtest / simulation / notebook evidence (if applicable): N/A - visually verified the new diagram renders correctly and depicts parallel trends / counterfactual / DiD effect accurately

Security / privacy

• Confirm no secrets/PII in this PR: Yes

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Generated with Claude Code

https://claude.ai/code/session_01QVCWFwqWfQWwSLB6fwcCzm

[github-actions]

Overall Assessment

✅ Looks good

Executive Summary

• Reviewed only the submitted binary asset change: diff-diff.png.
• No estimator, math, weighting, variance/SE, identification assumptions, or defaults are changed.
• The new diagram is consistent with the registry’s basic DiD definition: parallel pre-trends, treated counterfactual following the control trend, and the DiD effect as the treated-vs-counterfactual post-treatment gap.
• README continues to reference the same image path at README.md:L4-L6; image dimensions remain compatible with the fixed display width.
• No secrets or text metadata were found in PNG text chunks; the small EXIF/iDOT chunks appear limited to image metadata.

Methodology

No findings.

Severity: P3 informational
Impact: The image is documentation/branding only and does not alter any estimator behavior. The depicted basic DiD relationship matches docs/methodology/REGISTRY.md’s DifferenceInDifferences ATT formula.
Concrete fix: None required.

Code Quality

No findings.

Severity: P3 informational
Impact: No code paths changed.
Concrete fix: None required.

Performance

No findings.

Severity: P3 informational
Impact: The PNG is ~1.8 MB, which is acceptable for a README hero asset and unchanged in display behavior.
Concrete fix: None required.

Maintainability

No findings.

Severity: P3 informational
Impact: The existing README image URL and dimensions remain stable at README.md:L4-L6.
Concrete fix: None required.

Tech Debt

No findings.

Severity: P3 informational
Impact: No new deferred work or TODO tracking needed for this asset-only change.
Concrete fix: None required.

Security

No findings.

Severity: P3 informational
Impact: No obvious secrets, URLs, tokens, prompts, or text metadata found in the PNG.
Concrete fix: None required.

Documentation/Tests

No findings.

Severity: P3 informational
Impact: Binary-only documentation image change; automated tests are not necessary. Visual inspection confirms the diagram renders and aligns with the README context.
Concrete fix: None required.