feat: add Pipeline grouping and PipeInterface

[DerManoMann]

Summary

Enhances the existing Pipeline utility with ordered group execution, enabling staged processing pipelines (e.g. resolve → reduce → augment). Adds PipeInterface as an optional contract for
pipes that declare their group.

Depends on zircote#2054.

Design

Grouped execution

When constructed with a groups array, the pipeline executes pipes in group order rather than insertion order. This enables multi-stage processing where pipes from different groups can be
registered in any order but always execute in the correct sequence.

$pipeline = new Pipeline(
    [$augmentPipe, $resolvePipe],
    groups: ['resolve', 'reduce', 'augment'],
    defaultGroup: 'augment',
);
// resolvePipe runs first, augmentPipe second — regardless of insertion order

PipeInterface

Optional interface for pipes that declare their group:

interface PipeInterface
{
    public function group(): string|\BackedEnum;
    public function __invoke(mixed $payload): mixed;
}

Pipes without PipeInterface are placed in the pipeline's default group. Both string and BackedEnum group keys are supported.

Other additions

- get(class-string): ?T — retrieve a pipe by class for typed configuration access
- LoggerAwareInterface support — logger injected into pipes during process()
- walk() now reflects grouped execution order (not insertion order)
- process() uses ?? (not ?:) to preserve falsy return values from pipes

Backwards compatible

Without groups configured, insertion order is preserved — existing behavior unchanged.

[DerManoMann]

Moved to zircote#2056 (base branch updated to master now that spec-dto is merged)