gsp-phys: P6b waves 1.5 and 2 - deferred release and hardware-authored guest copies

- new default-off gsp_phys_work_defer_release: a forwarded guest push's
  releases are recorded and its GPFIFO entry is held (no consume, no
  GP_GET write, no counter publish/stall) until the work item publishes
  completion; the service thread then delivers the releases, with a
  fallback on failure/timeout (500 ms cap enforced every pump pass)/
  pause/quiesce/channel-delete
- new default-off gsp_phys_work_hw_primary: the software copy is skipped
  for forwarded ops, so the physical engine authors the data; a once-only
  software copy runs before the releases on any hardware failure or when
  the request cannot be queued
- release-apply helpers extracted; exactly-once latches; channel-delete
  and teardown handling; counters and a work_guard state line
- live: deferred-release run engaged=75 applied=75 timeouts=0 with the
  full smoke green; hardware-primary run req=100 ok=100 verify_ok=100
  fallback=0 copy_skipped=119 copy_fail=0, smoke ALL OK 64/64 (guest read
  data the engine authored), with 19 fail-closed inline software
  fallbacks under single-inflight contention; a range-unregistered run
  proved the fallback (121 skipped copies, all recovered, smoke green)
- defaults off; the software path and all existing guards unchanged

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