Bump version to 2.6.0

[igerber]

Summary

• Bump version from 2.5.0 to 2.6.0 across all version files (__init__.py, pyproject.toml, rust/Cargo.toml)
• Add CHANGELOG entry for v2.6.0 covering ContinuousDiD estimator, Stacked DiD tutorial, bootstrap NaN fixes, and anticipation parameter fix

Methodology references (required if estimator / math changes)

• N/A - no methodology changes (version bump only)

Validation

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

Security / privacy

• Confirm no secrets/PII in this PR: Yes

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[github-actions]

Overall assessment: Needs changes

Executive Summary

• The 2.6.0 changelog lists multiple methodology-affecting estimator changes, but this PR only bumps versions; please confirm those fixes are already in the release code or revise the notes accordingly. (CHANGELOG.md:L8-L31)
• Version numbers are aligned across Python and Rust entry points. (diff_diff/__init__.py:L162, pyproject.toml:L5-L8, rust/Cargo.toml:L1-L4)
• No new code paths or tests are introduced in this PR; edge-case checklist is not applicable unless the changelog entries correspond to actual new code in this release.

Methodology

• Severity: P1 | Impact: The changelog claims ContinuousDiD control-mask changes for anticipation and multiple inference/SE fixes, but there are no estimator code changes in this PR. The Methodology Registry explicitly requires the anticipation-adjusted control mask (G > t + anticipation). If the release notes outpace the code, users may assume methodology changes that are not actually present. | Concrete fix: Verify the ContinuousDiD implementation and listed inference fixes are already merged for v2.6.0; otherwise remove or defer those changelog entries or include the missing code changes in this PR. (CHANGELOG.md:L8-L31, docs/methodology/REGISTRY.md:L425-L435)

Code Quality
No findings.

Performance
No findings.

Maintainability
No findings.

Tech Debt
No findings.

Security
No findings.

Documentation/Tests
No findings.