feat: add timeline visualization report

[alexshamrai]

Add a new timeline-report that renders a Mermaid Gantt chart showing test execution across threads/workers over time in GitHub job summaries.

Tests are grouped by threadId (falls back to filePath or a single "Tests" section) and displayed as horizontal bars on a time axis. Colors indicate status: green (passed), red (failed). Skipped/pending tests are excluded from the chart but shown in the collapsible details table with precise timestamps (HH:mm:ss.SSS) and durations.

Uses Mermaid base theme with explicit colors and white background for consistent rendering across GitHub light and dark themes.

New Handlebars helpers: sanitizeMermaid, mermaidStatus, hasTimelineData, groupTestsForTimeline, groupAllTestsForTimeline, formatTimestampMs, timelineGroupCount.

Usage:
Action: timeline-report: true
CLI: npx github-actions-ctrf timeline --file report.json

[Ma11hewThomas]

I think this is a great idea, the report itself and using a mermaid gant chart. Design looks nice too!

How would it behave for large test runs? Should we add a max test count?

[alexshamrai]

I think this is a great idea, the report itself and using a mermaid gant chart. Design looks nice too!

How would it behave for large test runs? Should we add a max test count?

It should be tested for different setups, which I haven't done, yet: Long, short, rerun flaky tests etc. This is still a draft, that's why i created an issue #278 , to collect some feedback, and enhance requirements.

Not using mermaid there is an obvious issue, when test is failed on initialization and has very short duration, it is difficult to visualize.

For now, let's proceed with this design. Once I test more edge cases, I'll mark this PR as ready for review and tag you. Thank you!