feat: restart active performance traces

[godququ5-code]

Summary

• stop any currently running performance trace before starting a new one
• add a short notice when an existing trace is replaced
• cover the restart path with a regression test

Fixes #808.

Validation

• npm run test tests/tools/performance.test.ts
• npm run build
• npm run check-format

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[OrKoN]

Thanks for the PR but we are still collecting feedback on the linked issue.