Improve zero trust maturity floor gates

[danyili2632]

Summary

• add a weakest-pillar maturity floor so the overall rating cannot be inflated by one strong pillar
• add cross-pillar dependency risk scoring for attack paths created by maturity gaps
• add device compliance enforcement depth fields for grace periods, path coverage, session revocation, and remediation SLA evidence
• add legacy zero-trust readiness guidance for modern auth compatibility, segmentation, monitoring, and migration plans

Bounty

Addresses #1533.

I have read and agree to the CONTRIBUTING.md bounty terms. Preferred payment method can be provided privately after maintainer acceptance.

Validation

• git diff --check