vgsp: verify the scrub ring ledger before authoring (P0 for the X/GL wedge, CONT.427)

- Resolve the channel GPFIFO ring page through the GPU page tables (the
  same resolver the stash geometry path trusts) instead of deriving it as
  "hsem page - entries*8"; that subtraction is only correct when the
  buffer's GPFIFO and sema pages are PA-contiguous, and it could land on a
  stale or dense page (r1775/r1777 boots).
- Validate the first read with the strict GPFIFO entry shape (flag bits,
  length field, push VA bounded around the channel's own gpfifo) plus a
  clean zero terminus (the next slot and the following 32 slots must be
  zero). An all-zero page, a dense data page or a dirty tail is not
  trusted as a submission ledger.
- Detect a guest scrubber reset by a regressed finish word (or fin
  returning to 0 while a release was authored) and reseed the ledger
  instead of re-applying the pre-reset absolute count - that
  re-application underflowed the guest's scrub-list size (r1775 c6:
  4076 into an empty list -> 0xFFFFFFFFFFFFF014 -> the
  scrubCheckAndWaitForSize spin, _nv052600rm+0x7b).
- Never author the finish word from an unverified or unresolved ring:
  fin stays a no-op. A blocked scrub wait is recoverable; an underflowed
  list is not. A/B: sema_scrub_ring=0 skips the ledger entirely.

Live validation of this build is pending (6-cycle wedge acceptance +
GZDoom run).

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