#955 overlaps several open PRs. Resolution:
- GuestPageTracker, GuestBufferCache, ShaderIdentity, SharedBackingViews: upstream's versions
(they carry the same allocation-free invalidation, hashing and lock-free reads as #944/#968).
- RELEASE_MEM: upstream's deferred release flush (supersedes #966).
- CommandStreamQueue.Done: ours (#941 queued frame boundary; upstream only reformatted).
- Flip snapshot pool: ours (locked; snapshots are taken on the command-stream thread and
retired on the render thread), upstream's unlocked pool dropped.
- Gen5SpirvTranslator: our per-register variables and structurer (#923, #975/#976) first,
upstream's guarded in-order blocks as the fallback, then the dispatcher; upstream's
lane-spill slots, bounded move-relative, exec-known-full and PS input sharing kept.
9e1e15cc5c