Bump version to 2.8.0

[igerber]

Summary

• Bump version from 2.7.6 to 2.8.0 across all version locations
• Add CHANGELOG entry covering PRs Survey Phase 7: CS IPW/DR covariates, repeated cross-sections, HonestDiD survey variance #240, Thread styling kwargs through plotly visualization backends #244, Add Staggered Triple Difference estimator #245 and related work:
  • Staggered Triple Difference estimator (Ortiz-Villavicencio & Sant'Anna 2025)
  • Survey Phase 7a: CS IPW/DR + covariates + survey (DRDID nuisance IF corrections)
  • Survey Phase 7b: Repeated cross-sections (panel=False)
  • Survey Phase 7d: HonestDiD + survey variance
  • Plotly visualization styling kwargs
  • AI agent discoverability for practitioner guide

Methodology references (required if estimator / math changes)

• N/A — version bump only, no methodology changes in this PR

Validation

• Tests added/updated: No test changes (version bump only)
• Backtest / simulation / notebook evidence: N/A

Security / privacy

• Confirm no secrets/PII in this PR: Yes

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[github-actions]

Overall Assessment

✅ Looks good

Executive Summary

• The diff is a release/version bump only. No estimator, weighting, variance/SE, identification, or default-behavior code changed.
• The methodology-sensitive items mentioned in the new release notes are already covered by existing registry entries/notes for Callaway-Sant'Anna survey/RC behavior, StaggeredTripleDifference, and HonestDiD survey variance: docs/methodology/REGISTRY.md:420, docs/methodology/REGISTRY.md:426, docs/methodology/REGISTRY.md:1263, docs/methodology/REGISTRY.md:1308, docs/methodology/REGISTRY.md:1812.
• Version metadata is consistent across diff_diff/init.py:212, pyproject.toml:7, rust/Cargo.toml:3, and docs/llms-full.txt:5. A repo search did not find stale 2.7.6 references.
• The new release section and compare link are present at CHANGELOG.md:10 and CHANGELOG.md:1047.
• One documentation-only issue remains: CHANGELOG.md:31 appears duplicative of the earlier AI-discoverability note at CHANGELOG.md:54.

Methodology

• Severity P3. Impact: Informational only. The changed changelog summarizes methodology work already documented in the registry, and this diff itself does not alter estimator math, weighting, SEs, assumptions, or defaults. Concrete fix: None.

Code Quality

• No findings.

Performance

• No findings.

Maintainability

• No findings.

Tech Debt

• Severity P3. Impact: Informational only. The touched AI-guide file docs/llms-full.txt:5 is outside current doc-snippet smoke coverage, but that gap is already tracked in TODO.md:86. Concrete fix: None for this PR.

Security

• No findings.

Documentation/Tests

• Severity P3. Impact: CHANGELOG.md:31 appears to restate AI-agent discoverability work already captured in CHANGELOG.md:54, which makes the 2.8.0 notes ambiguous about what is newly shipped in this release. Concrete fix: Remove that bullet or replace it with the specific 2.8.0 practitioner-guide/discoverability change.