sema: never execute work decoded from swept RAM; pin only tracker sems

The content sweep reads arbitrary guest RAM and ran the full push decoder on
every page, so fabricated method streams in unrelated data executed
fills/copies: one boot scanned 65M pages, 'decoded' 124k pushes and ran
dozens of memsets from false positives - writes into live guest memory (a
guest kernel crashed in nvidia_uvm right after such a boot).

- Swept pages now only permit the bounded semaphore-release writes; fills
  and copies execute only for pushes the pump read through a trusted ring
  translation.
- Pushes read for a compute (GR) channel apply releases only: their method
  space is not the copy engine's, so aliased encodings must never become
  memory writes.
- sempages.py derives the pages to pin from the client's tracker objects
  (sem object + sem memory) instead of matching bare sem-object shapes, which
  pinned 140 pages where the client has ~53 sems; shape scan remains as a
  fallback when no tracker resolves.

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