Harden shutdown, locks, pipes, and child process handling

[foundev]

Problem

Host/editor disconnects, lock poisoning, closed pipes, and child shutdown need deterministic settlement and bounded progress.

Scope

• Audit mutex poisoning and convert it to bounded explicit errors.
• Test reader-thread exit with commands pending.
• Test closed editor stdout and closed host stdin.
• Reap the child and drain/capture stderr without deadlock.
• Add timeout protection to every shutdown path.

Acceptance

• Host or client disconnect settles every open ACP request.
• Shutdown cannot block indefinitely on I/O or lock ownership.

Source

• ROADMAP.md § “Shutdown and lock robustness”

[foundev]

Partially landed: 6753b1b (mutex poisoning recovery), bef105e (durable-host restart with session re-attach), and the client-disconnect coverage in 8d7aae9. Remaining: a bounded shutdown timer around child reaping and the closed-stdout/closed-stdin pipe tests. Narrowing this issue to that remainder.