M2 Phase 3: Wire Gate 5 into rebuild receipts

[gowdaharshith1998-lang]

Summary

• Wire gate5_property into the rebuild receipt path so gate 5 emits real passed/failed/runtime_crash/skipped/inconclusive statuses.
• Add --skip-gate-5 CLI plumbing with signed receipts still verifying.
• Preserve gate 6 as skipped/gate_not_wired and update M1 demo docs for M2-active gate 5.

Verification

• pytest tests/ -q (726 passed, 6 skipped, 27 xfailed)
• pytest tests/gates/ tests/rebuild/ tests/semantic/java/test_vendor_integrity.py -q (70 passed, 5 xfailed)
• python -m ruff check src/omnix/gates/gate5_property.py src/omnix/rebuild/runner.py src/omnix/cli.py tests/gates/test_gate5_property.py tests/rebuild/test_runner.py tests/rebuild/test_cli.py
• python -m mypy src/omnix/gates/gate5_property.py src/omnix/rebuild/runner.py src/omnix/cli.py
• bash -n docs/m1_demo_rehearsal.sh src/omnix/semantic/java/jvm/build.sh
• git diff --check
• npx gitnexus detect-changes --scope staged: HIGH risk, expected; affected rebuild run + gate5 check flows only

Notes

• Stacked on PR M2 Phase 2: Gate 5 Java property harness #22 because Phase 1 and Phase 2 are still open.
• Phase 3 HALT reached; continuing only because operator asked to finish the remaining dispatch as stacked PRs.