sema: r384b narrow the fill pin-demand hold to SYS (guest RAM) resolutions + FILL-EXEC witness

- The r384 boot held a large fill whose walk resolution was VID (the
  emulated vidmem shadow) - the hold was pointless overhead there and
  the re-decode re-resolves (the resolver is non-deterministic), so the
  hold now applies only when the fill would land in guest RAM (SYS
  aperture), the class that zeroed the guest's page-table pages. Shadow
  resolutions stay immediate.
- FILL-EXEC witness (bounded 64/boot): va, pa, aperture, provenance and
  pattern of every >= 4 KiB virtual fill execution - the decisive line
  for the boot-corruption forensics; the large zero fills were invisible
  until now.

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