vgsp: direct-GL ring scan hardening + USERD orientation overrides

- per-rank GR shadow-scan cadence (the global 1.5 s window was consumed by
  the first channel in pump order, starving the direct-GL ring scan)
- low-aperture ring content gate: entry0 within +/-8 MB of the ring VA and
  a zero terminus within the first 128 slots (dense data pages and transient
  copies were being latched as the ring and fed phantom entries)
- stop skipping the guest-RAM content scan when the shadow-scan window is
  closed; only the shadow scan is rate-limited now
- strict shared-surface author: write lastKickoffRefCount only, never the raw
  payload into an unvalidated translation
- dev-only sema_userd_swap / sema_userd_override to test the direct-GL USERD
  orientation and a known control page; both default off

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