[shossain786]

Closes #165. This is the remaining idea from the issue; the flaky trend line (#185) and hiding the empty flaky chart (#243) already landed.

What

• New opt-in flakiness.keepReports: N (default 0, so nothing changes unless it's set).
• After each run, the HTML report is copied into the history directory next to its metrics JSON as selenium-boot-report-<timestamp>.html. Copies beyond N are deleted, oldest first.
• On the trend charts, a point whose report was kept opens it in a new tab. Its tooltip says "Click to open this run" and the cursor becomes a pointer. Points without a kept report behave as before.
• The link is relative from the report to the history directory, so historyDir can point outside target/. When the two are on different roots, the link is a file: URI. A kept copy sits in the history directory, so its own links are plain file names.
• Documented in reporting/dashboard-trends.md.

Why opt-in

Reports embed their screenshots as Base64, so keeping every one by default could grow the history directory quickly.

Verification

• mvn clean verify: green.
• HtmlReportGeneratorTest: 37/37, including 4 new tests:
  • only runs with a kept report get a link
  • each copy gets local links, and the oldest copies are pruned
  • nothing is kept by default
  • the link base is relative for target/metrics-history and for ../.selenium-boot-history
• Browser check on a real generated report served over HTTP, with history in a sibling directory:
  • hovering a kept run shows the tooltip footer
  • clicking it requested ../hist/selenium-boot-report-20261110-100000.html (200)
  • clicking the current run's point did nothing

Not covered

• A real end-to-end run with keepReports set in selenium-boot.yml. The YAML binding is the same bean setter as historyRuns, and ExecutionMetrics.lastHistoryName() is set during export.

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