js-debug child-session adoption is not death-aware (burns full timeout windows against a dead child)

[debugmcpdev]

Problem

Found while tracing #242: once a js-debug child target dies mid-adoption, every remaining adoption step runs to its full timeout against a dead client instead of aborting. None of these hang start_debugging anymore (#242 fixed the barrier), but they keep the worker busy long after the debuggee is gone and delay teardown.

Specifics (all in src/proxy/child-session-manager.ts unless noted):

• attachChild (:346-369): 20 retries × 20s per-request timeout + 200ms sleeps — worst case ~400s — with all errors swallowed, no death check, no total deadline. A dead socket collapses to ~4s (sync throws), but a live js-debug socket that simply never answers attach for a vanished pending target burns the full ~400s.
• waitForEvent (:459-490) resolves false on timeout only and never observes client death; ensureChildStopped (15s + 5s threads + 30s pause), initializeChild (12s), handlePostAttachInit (3s) all burn their full windows once the child is gone.
• The close handler (:447-453) clears activeChild/childSessions but does not abort an in-flight createChildSession — the remaining steps keep running against a dead client.
• ChildSessionManager.shutdown() (:502-517) is never called from src/; MinimalDapClient.shutdown() only iterates children registered via childCreated (adoption success), so a child socket created mid-adoption is orphaned at worker shutdown.

Suggested direction

Thread a death signal (client close/error, or an AbortSignal) through createChildSession so each step (and the attachChild retry loop) bails immediately when the child dies; register the child socket for shutdown at creation time, not adoption success; give attachChild a total deadline.