vgsp: fail4 - route pending-pass releases through opdef withholding (cold-run gap)

The fwd-defer arm took priority over svc_opdef_release, so a pass whose op
was still pending recorded its fence release in svc_fwd_rel (applied at the
hold break / fallback) instead of the opdef slot - the fence fired while
the copy was still owed, the driver refilled the ping-pong staging, and the
late capture/read got refilled bytes (cold-run vector_add gap: one to four
chunks per cold boot). svc_fwd_defer_arm() is now skipped when
svc.op_tr_pending, so those releases go through svc_opdef_release and apply
only when the retried pass has no pending op (the op has run by then).

Measured cold-run vector_add bad across this iteration: 193536 -> 100352
(r323) -> 479232 (r324 wrong queue-pending experiment, reverted) -> 66560
(r325, ~1 chunk). All other acceptance items green on r325 (mail OK, smi OK,
matmul OK, transformer OK).

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