Stop exact descendants during pty kill

[myobie]

A background descendant that currently survives pty kill will now stop.

Summary

• Snapshot the exact descendant tree before the PTY child exits.
• Bind each PID to its process-start identity before each signal.
• Send exact TERM signals, then exact KILL signals after a bounded wait.
• Return a nonzero error if the daemon remains alive after seven seconds.
• Add the installed pty-kill-releases-socket-test command.

The shutdown never signals a process group.

Proof

• The current Darwin CLI failed the command and left the socket owner alive.
• The fixed Darwin CLI passed the kill-then-start cycle.
• The current Linux CLI failed the command and left the socket owner alive.
• The fixed Linux CLI passed the kill-then-start cycle under its subreaper.
• Focused tests passed 24 of 24 tests.
• The full local suite passed 1,615 tests. Two existing Nano tests failed because the host terminfo entry is incomplete.
• The build, type check, and documentation verification passed.

Nathan remains the merge gate.