vgsp-gr: m156 fail-closed release gate - backend half (witness + deferred release0)

The engine's QMD/channel completion ack is a front-end retirement, not an
execution witness. r155 live evidence: streams2's launch logged
qmd_rel=6/ch_rel=0xc0ffee02 while the retirement wait saw '0/205 pages
changed' on the initial attempt and all three outcome retries; the backend
then returned success and the client read all-zero output (STREAMS
bad=32768). The r152 matmul/smiley boots show the same signature.

For a data-bearing launch (resolved operand pages + working bounce window):

- new param gsp_phys_rmcall_gr_failclosed (default 1) forces release0 to
  the module semaphore (the m64 rel-mod shape), so the engine's ack cannot
  reach the guest's release word before an executor is witnessed;
- when the retirement wait still saw zero writes after the bounded
  retries, the backend returns -EIO instead of success and logs
  'FAIL-CLOSED: ... success withheld', so the sema service can withhold
  the guest release and fail loudly (companion commit).

Set gsp_phys_rmcall_gr_failclosed=0 to restore the historical fail-open
behavior. Build: make -j8 (kernel), green.

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