bf92727c9d359309c2a437a3b0f93d83e1fed73f  Implement the cross-origin-isolated permissions policy feature

Classified as not security-relevant.
Triage note: Splits crossOriginIsolated() into isInCrossOriginIsolatedAgentCluster() (used for agentClusterID/originAgentCluster, unchanged) and a new crossOriginIsolated() that also requires PermissionsPolicy::Feature::CrossOriginIsolated, and registers that new 'cross-origin-isolated' permissions-policy feature. Additive web-platform/spec feature, not a silent vuln fix.
This commit was flagged by the signal scorer and reviewed by AI (not a human). It is listed only so the triage is auditable; there is no root-cause analysis because this is not a vulnerability fix.

Structural signals: smartptr_member_added (+3.0), area:Source/WebCore/dom (+2.0), surgical (+1.3)