[bugfix] Bound worker startup and propagate child failures

[XingLiu1]

Propagate TransferManager initialization tracebacks to the parent, bound the spawn handshake and SGLang readiness polling, and keep checking worker health after readiness and during completion waits. Preserve the caller's MPI initialization environment. Let multiprocessing collect child exit status instead of a generic SIGCHLD reaper, and close task-owned pipes during shutdown.

Clean the newly created HugeTLB file, mapping and descriptor when a shareable allocation fails. Existing graceful transfer-drain/shutdown timeouts remain in force. The new timeout settings are documented in the configuration reference.

Validation: 13 CPU regressions passed, including a real child initialization exception, child exit during an unbounded wait, invalid/deadline configurations, environment restoration and injected mmap/wrap failures. Applicable pre-commit hooks passed. No GPU/DMA or real HugeTLB allocation acceptance is claimed.