feat: complete Nexus Unity 1.7.0 cleanup pass

[Daliys]

Summary

• Complete the Nexus Unity 1.7.0 cleanup pass on the existing capture work.
• Add driver-owned async capture, canonical command/runtime structure, optional Pipeline support with Legacy compatibility, and canonical Tests/Editor coverage.
• Align package/docs/release metadata with the 1.7.0 development line and latest released v1.6.0.
• Add the public BENCHMARKS.md ledger with verified validation evidence and explicit TBD numeric performance fields.

Validation

• Static pre-push validation: 43 Python bridge tests passed; quality gate 0 errors and 5 warnings.
• Interactive Unity EditMode: 152/152 passed, 0 failed.
• Clean batch Unity run: 151 passed, 0 failed, 1 inconclusive because visible Game View is unavailable in batch mode.
• Domain reload: 20/20, 0 hangs.
• Multi-editor validation: Pipeline ports 7800/7801 with no Legacy 8081 conflict.
• Clean compilation passed with and without Pipeline; capture smoke passed for 1600x900 PNG/JPEG.

Notes

• Latest released version is v1.6.0; this branch is the 1.7.0 development line.
• Legacy transport remains available because Pipeline is optional.
• No unverified latency, CPU, or memory claims are presented; those remain TBD in BENCHMARKS.md.
• The pre-push live smoke was skipped because no local server was listening on 127.0.0.1:8081.