Add benchmark documentation and middleware benchmark coverage, fix the settlement benchmark mock to satisfy the current SettlementStore interface, and add reusable domain test helper packages. Verified with fresh go test runs for ./internal/testutil/... and go test -tags=slow -run '^$' ./internal/benchmark/... before commit.
Add nil and wildcard coverage for IAM claims helpers, ensure auth security tests run with authentication enabled, and make settlement mocks express pending-withdraw/error branches explicitly.