Job control & concurrency: Remove race between SIGCHLD handler and job table updates

[exlier]

Problem

• Job table updates race with SIGCHLD delivery and the main loop, causing inconsistent job states (jobs shown as running after exit, jobs not removed, or duplicate entries).

Impact

• Users see stale or incorrect job statuses; fg/bg/kill sometimes acts on wrong PIDs. Race conditions can also lead to inadvertent killing of the shell's process group.