Run PIT mutation testing on self-hosted performance runner

[astubbs]

Context

PIT mutation testing on GitHub's ubuntu-latest runners is extremely slow:

• Coverage phase: ~7 minutes (fine)
• Mutation phase: hit GitHub's 6-hour per-job hard limit without completing (76 mutation units targeting io.confluent.parallelconsumer.*)

Narrowed to internal.* classes only and added incremental history caching in PR #34, which should help. But the long-term fix is running PIT on the self-hosted performance runner where CPU and I/O are much faster.

Proposal

Move the mutation-testing job to runs-on: [self-hosted, performance] (same runner used by the performance tests workflow in .github/workflows/performance.yml). This would let us:

• Target the full io.confluent.parallelconsumer.* class set
• Complete in minutes instead of hours
• Potentially make PIT a blocking check again

Current workaround

PIT runs on ubuntu-latest with continue-on-error: true (non-blocking), timeout-minutes: 300, targeting only internal.* classes, with incremental history cache.

See docs/SELF_HOSTED_RUNNER.md for runner setup.