INT-OBJECT-01H: Refuse deferred advertised class identities #257

Problem

The finite construction profile permits lexical class-cell interactions with a class namespace named class, but custom advertised-class lookup remains deferred. Attribute reads, failed actual-type isinstance checks and super receiver validation can currently return misleading values instead of reaching that explicit boundary.

Scope and acceptance

Under #123, refuse these deferred lookups before descriptor callbacks. Preserve type(value), successful actual-type checks and real-subtype super binding. Add authored CPython references, explicit Future classifications, source/ownership regressions, Node/Chromium drain/step and offline acceptance, with exact existing composed resource baselines. No new protocol, host capability, dependency, publication or deployment is included.

Depends on #251 and PR #256. Use a separate focused correction PR.