feat(gates): add gate6 probe harness

[gowdaharshith1998-lang]

Summary

• Adds Gate 6 dual-runtime probe orchestration with isolated JVM subprocesses per side/probe.
• Adds the vendored EquivalenceProbeRunner jar and build.sh smoke coverage.
• Covers R-4.2 crash isolation, R-4.3 timeout handling, R-4.4 deterministic repeat observations, and R-4.5 StringUtils.reverse identity probes.

Verification

• python -m ruff check src/omnix/gates/gate6_equivalence tests/gates/test_gate6_harness.py tests/semantic/java/test_vendor_integrity.py
• python -m mypy src/omnix/gates/gate6_equivalence
• bash src/omnix/semantic/java/jvm/build.sh
• pytest tests/gates/test_gate6_harness.py tests/semantic/java/test_vendor_integrity.py -q
• pytest tests/gates/ tests/semantic/java/test_vendor_integrity.py -q
• pytest tests/ -q --tb=line
• npx gitnexus detect-changes --scope staged: low risk, 0 affected processes

Notes

• Stacked on Phase 3 PR because earlier M2 PRs are still awaiting merge.
• GitHub Actions may not start due the known account billing/spending-limit block documented in the dispatch.