Document stacking architecture and implementation plan

[imakris]

Summary

• add the source-grounded architecture and stacking review
• add the five-stage implementation plan and executable batch gates
• record owner approvals for D1-D3 and D5-D10, leaving D4 open for A/B evidence
• record API-break and branch/PR authorization plus calibration, artifact, backend, and review automation contracts

Current gate evidence

• existing initialized Release CTest: 21/21 passed
• canonical Varinomics style pipeline executes and confirms the documented 14-violation Batch 1B baseline
• git diff --check: clean
• dependency Windows startup repair is present at vnm_msdf_text@b9c216a; its normal Windows job passed and its full-export job was still running when the execution record was written

This PR contains documentation only and does not begin product stack implementation.