Release v0.4.0

What's Changed

• docs: include implementation refactor in v0.4.0 changelog
• docs: extend v0.4.0 changelog with PR #26 compliance fixes
• docs: add v0.4.0 changelog entry
• refactor IXDTF implementation by concern
• Potential fix for pull request finding
• Potential fix for pull request finding
• Potential fix for pull request finding
• fix: resolve RFC 9557 compliance findings from expert review
• test: cover internal branches flagged by codecov
• style: resolve gocognit and nestif lint findings
• docs: align comments and README with critical time-zone semantics
• test: broaden critical time-zone coverage
• fix: reject CriticalLocation without Location in extension validation
• fix: stop skipping the consistency check for Etc/GMT zones
• fix: accept offset-derived time zones in consistency and strict checks
• fix: reject a second time-zone annotation instead of overwriting the first
• fix: accept critical "!" flag on time-zone annotations per RFC 9557
• fix: treat Z/-00:00 as unknown local offset per RFC 9557 (#22)
• ci(deps): bump actions/cache from 5 to 6
• ci(deps): bump actions/checkout from 6 to 7
• ci(deps): bump codecov/codecov-action from 6 to 7
• ci(deps): bump codecov/codecov-action from 5 to 6
  Full Changelog: v0.3.1...v0.4.0