refactor: add maxDelay param to calculateDelay and export MAX_TIMEOUT_DELAY

Export MAX_TIMEOUT_DELAY as a named constant (2^31 - 1) so callers can
reference the same ceiling used internally. Add an optional `maxDelay`
param to `calculateDelay` so callers can cap the backoff independently of
the hard setTimeout ceiling. Add unit tests covering exponential growth,
the custom cap, and the absolute ceiling.