Improve RBAC role mining evidence

[DENGXUELIN]

Summary

• Adds a role-mining dataset quality gate to rbac-design.
• Requires source coverage, extraction date, observation window, entitlement normalization, account filtering, permission-use evidence, owner validation, and outlier remediation before mined roles are promoted.
• Adds vulnerable and benign fixtures for dirty versus validated role-mining datasets.

Issue

Fixes #1699

Validation

• git diff --check origin/main...HEAD
• Markdown fence balance check
• Added-line ASCII check
• Content marker check for RBAC-MINE-* and fixture expectations
• git merge-tree --write-tree origin/main HEAD

Bounty

Requested tier: Improver Moderate ($100) if accepted/merged.