Clarify posture readiness docs

[SSBrouhard]

Summary

• clarify README posture language and v0.3.0 design-vs-implementation boundary
• add posture evidence requirements to the deployment runbook, evidence notes, release checklist, and DSOVS mapping
• update the changelog for the posture self-check and v0.3.0 design records

Validation

• git diff --check
• .venv/bin/python scripts/vmga_release_check.py

Notes

• Docs-only change.
• During review I also observed three open CodeQL clear-text logging alerts in the GitHub API; those are separate from this docs pass and should be handled in a follow-up security PR.