feat: prefork child auto-restart on crash

[pratyush618]

Summary

• Child liveness check: Before dispatching each job, check if any child processes have died
• Auto-restart: Dead children are respawned with new reader threads, in-flight count reset
• Restart tracking: restart_count logged with each restart for monitoring

Test plan

• cargo check all feature combos
• cargo test --workspace passes
• Clippy clean