refactor: make TrackedChild easier to use

[wan9chi]

refactor: make TrackedChild easier to use

This PR refactors the TrackedChild struct to make it easier to use by:

• Adding a ChildTermination struct that contains both the exit status and path accesses
• Providing a wait_handle future that resolves to ChildTermination when the process exits
• Exposing stdin, stdout, and stderr handles directly on TrackedChild
• Automatically locking the IPC channel when the child process exits
• Updating all usages of TrackedChild throughout the codebase

[wan9chi]

• refactor: make TrackedChild easier to use #20 👈 (View in Graphite)
• main

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