test(e2e): add 2026-06 spec requirements for SEP-2260 and SEP-2575/2567 as expected-fail stubs

63 new requirements covering SEP-2260 (server-request association) and the
stateless/sessionless cluster (SEP-2575 + SEP-2567), each cited by a
deliberately-failing stub in scenarios/spec-2026-06-stubs.test.ts and marked
as a knownFailure. No SDK changes; matrix stays at 0 unexpected failures
(916 pass + 131 expected-fail). Sub-batch labels (P0, G0, S1-S9) give the
proposed implementation order.

Author: felixweinberger