processor.Channel reports undelivered items as successful on context cancel

[MasterOfBinary]

When the context is canceled mid-batch, processor.Channel.Process returns immediately with the remaining unsent items untouched (processor/channel.go:30-34):

select {
case <-ctx.Done():
    return items, ctx.Err()
case p.Output <- item.Data:
}

The stage error becomes a single ProcessorError, but every item that never reached Output comes back with Error == nil — per-item accounting says "delivered" for data that was never sent.

Repro sketch: unbuffered Output with a slow consumer, cancel the context after the first send; items 2..N are reported as processed successfully.

Suggested fix: mark undelivered items with ctx.Err() before returning (same pattern as processor.Nil's MarkCancelled), so downstream error accounting reflects reality.

Found in the 2026-07-10 full-repo review (verified by repro).