Order unified exec lifecycle events reliably

[copyberry]

Order unified exec lifecycle events reliably

What changed

• Treat output-task closure as the signal that trailing output is complete, drain any remaining chunks before publishing command completion, and retain the grace period as a fallback.
• Wait for deferred network-denial classification before emitting the final command result.
• Serialize write_stdin interaction and completion events so an interaction that exits a process is published first, and avoid pruning processes while their terminal events are being finalized.

Testing

• Add coverage for late output, missing output-close signals, late network denials, interaction/completion ordering, cross-platform aggregated output, and pruning during finalization.