Summary

Add progress + cancellation support to the batch primitives.

Motivation

Batch operations need to report rich progress and abort not only on request but also fail-fast when a non-TryEach delegate throws.

Scope

2.7.1 Cancellation — Fail-Fast

• If a non-TryEach delegate throws, the library signals cancellation to the remaining tasks via an internal CancellationTokenSource, preventing wasted work whose results will be discarded.
• External CancellationToken from the caller is honored.

2.7.2 Progress reporting

• IProgress<T> integration returning a rich ExecutionProgress, not just 0-100%:

public record ExecutionProgress(
    int CompletedCount,
    int TotalCount,
    int FailedCount,
    TimeSpan ElapsedTime
);

• ExecutionProgress record.
• Fail-fast CancellationTokenSource cascade on first error (non-TryEach).
• Caller-supplied CancellationToken honored throughout.
• Progress throttled/reported via IProgress<T>.
• Unit tests: fail-fast aborts siblings; external cancel aborts all.

Design Expectations

• Progress is rich, not a bare percentage.
• Fail-fast only applies to throw-style (non-TryEach) execution.
• Composes with concurrency/map primitives.

Acceptance Criteria

• On first throw, remaining tasks receive cancellation.
• Reported progress reflects completed/failed/total/elapsed.
• Caller CancellationToken stops the batch promptly.

Non-Goals

• No retry policy (Polly adapter).
• No partial-success aggregation (TryEach covers single failures without aborting the whole).