Service-atom chains (std-service-stripe createPaymentIntent →
PAYMENT_CREATED → confirmPayment → PAYMENT_CONFIRMED → succeeded)
stalled in playgrounds because mock mode had no default callService —
the runtime warned and returned null, breaking the chain at the very
first emit.success that downstream transitions need.

Now mock mode synthesises a sensible result `{id, clientSecret, success,
status, ...params-echo}` covering the common service-result shapes so
state machines progress end-to-end without per-service config. Real
mode still warns and returns null when callService isn't configured.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>