`atomic` is the `atomic_batch` capability (§75.3, §67.4), and a runtime
that does not advertise it must refuse the request with
UnsupportedCapability rather than run it as a sequence. The Rust engine
does (`run_prepared_request`, pinned by
`atomic_execution_is_refused_rather_than_faked`); kip-do answered
UnsupportedIsolation, so the two engines disagreed on the code. The HTTP
status is unchanged (both codes are protocol / non-retryable, 400 here).

An isolation level the engine cannot provide still answers
UnsupportedIsolation (§32.2), as the shared request-envelope fixture pins.
request.test.ts asserts both refusals; the Durable Object test now expects
UnsupportedCapability.

Checks: kip-do tsc --noEmit and vitest run (workerd pool, 577 passed);
cargo test -p anda_kip.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>