vgsp-sema: m159 wait+retry for busy-refused GR captures (replaces hold-and-reprocess)

r159: the m156/m158 hold kept the entries but the pump never re-processed
them before the client tore down (held entry re-decode is a replay; the
one-shot window and teardown timing conspired), and the channel died with the
launches unforwarded.

New design: on a busy refusal with the bridge disabled the capture is NOT
consumed and the entry loop waits (bounded 4 s) for the slot to free -
resolving the active release hold through its own channel on this same pump
thread - then retries svc_fwd_request inline. Multi-launch clients now
serialize correctly: kernel 2/3 of streams2 wait for kernel 1's forward and
then run. On timeout the historical consume+soft-release path stands.

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