feat(gates): wire gate6 classifier into receipts

[gowdaharshith1998-lang]

Summary

• Adds Gate 6 probe classification and overall evaluation for all seven required buckets.
• Wires Gate 6 evaluation into rebuild receipts so all six gates now emit real statuses.
• Records passed/inconclusive/failed Gate 6 details, including probe_count, divergence_count, accepted_with_note_count, and diverging_input for failures.

EARS Coverage

• R-6.1: classify_results assigns exactly one bucket per probe.
• R-6.2: value, exception, and deterministic stdout/stderr divergences fail Gate 6.
• R-6.3: replay-based stochastic observable differences are accepted with note.
• R-6.4: double relative tolerance uses 2^-52.
• R-6.5: passed Gate 6 receipts include required counts.

Verification

• python -m ruff check src/omnix/gates/gate6_equivalence src/omnix/rebuild/runner.py tests/gates/test_gate6_classifier.py tests/rebuild/test_runner.py
• python -m mypy src/omnix/gates/gate6_equivalence src/omnix/rebuild/runner.py
• pytest tests/gates/test_gate6_classifier.py tests/rebuild/test_runner.py -q
• pytest tests/gates/ tests/rebuild/test_runner.py tests/semantic/java/test_vendor_integrity.py -q
• real Gate 6 evaluate on identical StringUtils.reverse sample: passed, 26 probes, 0 divergences
• pytest tests/ -q --tb=line
• npx gitnexus detect-changes --scope staged: MEDIUM risk, 4 rebuild flows affected

Notes

• Stacked on Phase 5 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.