Add terminate-on-parent-exit subprocess option

[sheredom]

Summary

• add subprocess_option_terminate_on_parent_exit to terminate a managed subprocess tree when its parent exits
• use a kill-on-close Job Object and atomic PROC_THREAD_ATTRIBUTE_JOB_LIST assignment on Windows 10+
• use a dedicated process group, parent-lifeline pipe, and watchdog on POSIX without moving the caller into the managed group
• make subprocess_destroy terminate opted-in running process trees
• document platform behavior and the POSIX process-group escape limitation
• add parent-exit, explicit-destroy, descendant, and failure-cleanup coverage

Rationale

The process-group design incorporates the lesson from Apache Commons Daemon DAEMON-232 and commit cab468ff, which removed jsvc's unconditional startup setpgrp/setpgid behavior because it interfered with supervisors that signal logical process groups.

The POSIX watchdog also closes its own lifeline write descriptor before waiting for EOF, matching the failure mode fixed by OpenJDK JDK-8307990.

Testing

• ctest --test-dir build --output-on-failure
  • subprocess_test
  • subprocess_address_test
  • subprocess_undefined_test
  • subprocess_thread_test
• MinGW x86-64 C compilation and C++ Windows-header include-order checks

All four local macOS CTest targets pass. Windows runtime and Linux behavior are covered by the repository CI matrix.

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