vgsp: GR data path - pre-publish bounce fill, engine-change proof, ipin-first data resolution

Two fixes and one diagnostic for the all-zero-results class (r89: smiley's
4 vadd launches reported GUEST GR LAUNCH OK yet every output word read 0):

- gsp_phys_booter: copy the guest data pages into the bounce window BEFORE
  gsp_phys_rmcall_publish_locked() rings the doorbell. The fill used to run
  after the kick; the engine front end starts within ~7 ms, so the kernel
  could read the window before it was filled.
- gsp_phys_booter: per-page checksums at copy-in, compared at copy-out;
  the copy-out log now reports 'N changed by engine'. An all-unchanged
  window after a reported completion is the front-end-ran/grid-never-
  dispatched signature (completion semaphores are front-end authored).
  The first 3 data (va,hpa,reloc) triples are logged per launch.
- gsp_sema_service: svc_gr_data_fill resolves data pages through
  svc_ipin_lookup() first (guest-truth pins), falling back to
  svc_resolve_va, mirroring the pool resolver order; a wrong page here
  silently runs the kernel on zeros. First 16 resolved (va,pa,ap) triples
  are logged for post-mortem comparison against the guest's own mapping.

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