Sync ROADMAP and TODO with v2.3.0 release

[igerber]

Summary

• Remove shipped Borusyak imputation subsection from ROADMAP (implemented in v2.3.0)
• Remove completed staggered.py/prep.py split item from ROADMAP Infrastructure section
• Remove 3 resolved sections from TODO (rank-deficiency handling, Rust Clippy, RuntimeWarnings)
• Bump ROADMAP version references: current → v2.3.0, near-term → v2.4, medium-term → v2.5+

Methodology references (required if estimator / math changes)

• N/A — no methodology changes, documentation-only

Validation

• Tests added/updated: No test changes (docs-only PR)
• 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: ✅ Looks good

Executive summary

• Docs-only updates: roadmap version markers bumped and completed items removed; no code or estimator changes in diff (ROADMAP.md:L11-L209, TODO.md:L13-L140)
• Current status now reflects v2.3.0 with near-/medium-term labels updated to v2.4/v2.5+ (ROADMAP.md:L11-L83)
• TODO cleanup removes resolved rank-deficiency/Rust/RuntimeWarning entries; remaining issues list is intact (TODO.md:L13-L140)
• Methodology registry reviewed for context; no methodology adherence checks triggered because estimators/SE/assumptions are untouched (docs/methodology/REGISTRY.md)

Methodology

• Severity: P3 | Impact: None (docs-only; no estimator/math/weighting/SE/assumption changes in diff) | Concrete fix: None

Code Quality

• Severity: P3 | Impact: None (no code changes) | Concrete fix: None

Performance

• Severity: P3 | Impact: None (no runtime changes) | Concrete fix: None

Maintainability

• Severity: P3 | Impact: None (documentation clean-up only) | Concrete fix: None

Tech Debt

• Severity: P3 | Impact: None (tracking docs updated; no new debt introduced) | Concrete fix: None

Security

• Severity: P3 | Impact: None (no secrets or code changes) | Concrete fix: None

Documentation/Tests

• Severity: P3 | Impact: None (docs-only; no tests expected) | Concrete fix: None