Phase-23 accepted-evidence set membership assignment decision

[kenanay]

Summary

Adds PHASE23_ACCEPTED_EVIDENCE_SET_MEMBERSHIP_ASSIGNMENT_DECISION.md as the Phase-23 accepted-evidence set membership assignment decision local draft based on exact-main subject 3c8cdfa992ae1c94098b75b7b6064bf8bd40c184 from PR #277.

This PR changes only PHASE23_ACCEPTED_EVIDENCE_SET_MEMBERSHIP_ASSIGNMENT_DECISION.md and accepts only a bounded accepted-evidence set membership assignment decision boundary. It does not assign accepted-evidence set membership, create an accepted-evidence set, create accepted evidence, accept any evidence item, accept validator output, accept receipt evidence, or authorize runtime/source/package/source-merge/capability/registry/trust/deployment/distribution/kernel ABI/syscall/workflow-threshold/baseline/dependency/Ring0 authority.

Scope

• PHASE23_ACCEPTED_EVIDENCE_SET_MEMBERSHIP_ASSIGNMENT_DECISION.md only
• No docs/roadmap/CURRENT_PHASE change
• No prior Phase-23 decision/candidate file changes
• No CI workflow, baseline, dependency, runtime/kernel source, syscall, or kernel ABI changes
• PHASE21_FIRST_BOUNDED_IMPLEMENTATION_ACTUAL_SKELETON_PR_DESIGN.md remains PR-disjoint

Validation

• git diff --cached --check
• Local staged scope confirmed before commit

[github-actions]

Ayken Advisory

• Authority: none
• Confidence: -
• Risk: high
• Nearest Verified Ancestor: -
• Distance: - commits
• Gates: PASS

Advisory only — does not override CI authority.

[kenanay2020-hub]

Approved: single-file assignment decision boundary scope, exact-SHA binding, and fail-closed non-authorization boundaries verified.