Add ThreadMine to Performance analysis

[maschiojv]

Adds ThreadMine under Performance analysis, next to fastThread.

It's a hosted JVM thread dump analyzer: paste or upload a dump (HotSpot jstack/jcmd, OpenJ9 javacore, Zing, GraalVM native-image) and get deadlocks, pool exhaustion, virtual thread pinning and a health score, plus side-by-side comparison of two dumps.

Commercial with a free tier: the first analysis needs no signup, and there's a Free plan. Pricing is public at https://threadmine.dev/en/pricing.

Disclosure: I'm the founder. This is a separate PR from #1322 (the open-source CLI that masks dumps before upload), one project per PR as the guidelines ask.

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Summary by cubic

Adds ThreadMine to the Performance analysis section of the README, next to fastThread.

• Describes a web-based JVM thread dump analyzer covering HotSpot, OpenJ9, Zing, and GraalVM dumps, with deadlock, pool exhaustion, and virtual thread pinning detection.
• Notes that it is commercial with a free tier where the first analysis needs no signup, and that the contributor is ThreadMine's founder.

^{Written for commit 38d36eb. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 1 file

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