Improve HIPAA audit-control coverage gates

[MAUROCERON]

Implements #1413.

Summary

• Adds a 164.312(b) audit-control evidence gate for ePHI system coverage, event taxonomy, log source, integrity/time basis, retention, and activity-review linkage.
• Adds an output matrix so HIPAA reviews can document audit-control coverage per ePHI system instead of accepting generic logging.
• Adds edge-case fixtures for login-only logging, mutable logs, missing activity-review linkage, missing Business Associate audit evidence, and complete coverage.

Validation

• Checked Markdown fence balance for the updated skill and new fixture.
• Verified required markers: Audit-control evidence gate, Event taxonomy, Integrity/time basis, Activity-review linkage, Not Evaluable, Business Associate.
• Verified official eCFR and NIST reference URLs return HTTP 200.

Payment details can be provided privately after maintainer acceptance.