Gatherer for sequence/traverse with true short-circuit

Context

sequence and traverse must implement fail-fast semantics: upon encountering the first Err/Failure they must stop processing elements. The Stream API does not allow short-circuit with a Collector.

Decision

Implement sequence/traverse using Stream.gather(Gatherer.ofSequential(...)) (Java 22+) instead of reduce, forEach, or Collector.

Consequences

Positive:

• True short-circuit: the stream is closed upon encountering the first error without consuming the rest.
• The code is declarative and composable within the stream pipeline.
• Gatherer is Java’s official abstraction for stateful transformations in streams.

Negative / tradeoffs:

• Requires Java 22+ (preview feature in 22, finalized in 24).
• Collector-based operations (toList(), partitioningBy()) continue consuming the entire stream — different behavior explicitly documented.

Alternatives considered

• Manual reduce: does not allow short-circuit in sequential streams without using exceptions as control flow.
• forEach with mutable state: works but is not idiomatic with streams and breaks composability.
• Custom Collector: cannot short-circuit by design of the Collector API.