Bump version to 3.4.2

[igerber]

Summary

• Bump version 3.4.1 → 3.4.2 across diff_diff/__init__.py, pyproject.toml, rust/Cargo.toml, diff_diff/guides/llms-full.txt, and CITATION.cff (date-released → 2026-05-25)
• Rename CHANGELOG.md [Unreleased] section to [3.4.2] - 2026-05-25 (pre-populated content preserved) and add the v3.4.1...v3.4.2 comparison link

Methodology references (required if estimator / math changes)

• N/A — version-bump only, no estimator / math / methodology changes

Validation

• No test changes (version-bump only)
• Pre-populated [3.4.2] CHANGELOG section captures the methodology work already merged on main since v3.4.1

Security / privacy

• Confirm no secrets/PII in this PR: Yes

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Generated with Claude Code

[github-actions]

Overall Assessment

✅ Looks good

No estimator, math, weighting, variance, assumption, or default-behavior code changes are introduced by this PR. I found one minor P3 release-surface inconsistency, but no unmitigated P0/P1 issues.

Executive Summary

• Affected methods: none. docs/methodology/REGISTRY.md and estimator implementations are untouched by the diff.
• The changed release surfaces are otherwise consistent: pyproject.toml, diff_diff.__version__, rust/Cargo.toml, CITATION.cff, diff_diff/guides/llms-full.txt, and the new changelog header all move to 3.4.2.
• P3: the spillover carousel generator still hard-codes v3.4.1, so regenerated collateral would ship the old footer label.
• No edge-case, inference, control-group, NaN/Inf, or parameter-propagation risks are introduced because no estimator/runtime code changed.
• No security or performance findings.

Methodology

• No findings. This is a metadata-only release bump; there is no affected estimator to cross-check against papers or the Methodology Registry.

Code Quality

• No findings. The runtime/package version surfaces changed by the diff are internally consistent.

Performance

• No findings. No executable estimator or backend logic changed.

Maintainability

• Severity P3 — carousel/generate_spillover_carousel.py:L14 and carousel/generate_spillover_carousel.py:L47 still advertise v3.4.1. Impact: if the carousel is regenerated after this release bump, the exported PDF will carry the wrong release label even though the package ships 3.4.2. Concrete fix: update both the docstring note and VERSION_LABEL to v3.4.2, or derive the footer from a shared version source to prevent future drift.

Tech Debt

• No findings. I did not see any new deferred correctness or methodology debt introduced by this diff.

Security

• No findings. The changed files are metadata/documentation only and I saw no secrets or new execution surface.

Documentation/Tests

• No findings requiring changes. Skipping runtime test edits is reasonable for a metadata-only release bump.