Cancel-safe S3 uploads implementation plan

Goal

Ensure cancelling an ordinary, non-resumable large-object upload converges to one bounded AbortMultipartUpload attempt instead of leaving an orphan.

Architecture

After multipart creation, start one metadata-only cleanup owner containing the S3 client and upload identity. A one-shot decision channel disarms it after successful completion or requests an awaited abort on explicit failure. If the caller future is dropped, channel closure selects the same abort path. The owner task lives with the active upload and caps its terminal abort attempt at 30 seconds; it never owns object buffers. Resumable uploads retain their checkpoint-owned lifecycle.

Tasks

1. Add a real LocalStack regression that cancels a blocked ordinary upload and observes ListMultipartUploads converge to empty.
2. Add unit coverage for drop, disarm, and explicit-abort owner transitions.
3. Route ordinary error/completion/cancellation through the cleanup owner while preserving part cancellation before abort.
4. Document the process-crash boundary and mandatory S3 incomplete-multipart lifecycle rule.
5. Run lane-1 lifecycle, full LocalStack, strict Clippy, rustdoc, full gate, and independent review.