Reduce resume handoff and shutdown delays

[fazxes]

Summary

• Confirm Ctrl+C resume handoffs without replaying saved history or generating a final checkpoint.
• Keep derived publication failures from forcing canonical session replacement or suppressing an otherwise valid handoff.
• Cancel picker loads when abandoned or during shutdown.
• Revalidate cached sessions when /resume opens instead of reusing a page for five seconds without refreshing.
• Reject session-index results when publication changes are detected during the read.
• Keep picker discovery from rebuilding indexes or writing derived session metadata.
• Report managed-child metadata errors instead of silently treating those sessions as hidden children.
• Force-stop managed commands during exit and stop draining their output after termination when an external process retains an output descriptor.

Known gaps

• Startup and picker-selected history restoration remain synchronous and cannot yet accept Ctrl+C during replay.
• The 500 ms shutdown ceiling is not established across every parser, replay, and cancellation path.
• Publication revalidation detects tested interleavings, but the full concurrent-writer epoch guarantee still needs verification.

This is a draft checkpoint, not a complete implementation of the shutdown contract.