feat: add Parallel.race() — first successful result wins

[praveer-rai]

Runs all suppliers concurrently and returns the first successful result. Remaining tasks are interrupted. If all fail, the last failure is rethrown.

var result = parallel.race(
    () -> fetchFromPrimary(),
    () -> fetchFromReplica()
);

Useful for hedged requests — try multiple paths, take whichever responds first. Supports varargs and List<Supplier<T>> overloads. Respects withMaxConcurrency and context propagation.

Test plan

- Returns first successful result, ignores slower tasks
- Interrupts remaining tasks after first success
- Succeeds if some tasks fail but at least one succeeds
- Throws when all tasks fail
- Rejects empty task list
- Propagates MDC context
- README updated

[jimbao]

Lets consider when a practical use case appears