SchemaRedefineSelfReference.swift had reached the 400-line limit, blocking further
redefine-restriction work without a squeeze. It also mixed three cohesive concerns:
redefine self-reference well-formedness (src-redefine.6/7.2.1), attribute-group
restriction (cos-ct-restricts over the redefinition), and model-group restriction.
Move the attribute-group restriction machinery (redefineAttributeGroupRestrictionFindings
and its helpers) into SchemaRedefineAttributeGroupRestriction.swift, following the
established one-concern-per-file pattern (IdentityFieldType.swift). The shared base-
component lookups (baseComponents/resolveBaseComponent) become internal so both files
use them. Pure code move, no behaviour change: 171 + 232 lines (was 400); full suite
and the XSTS gate pass unchanged (20/0/22/0 buckets, 1558 unit tests).
