Fix SimpleCPUOffload load-path race — add cross-stream sync (WAR)

[zaid646]

Fixes #47282

Root cause: After #46278 fixed the store-side compute-vs-store race, a second independent race remained on the load path. When a GPU block is freed and immediately reallocated as a load destination, the load DMA (on the dedicated low-priority load_stream) can begin overwriting that block while a previous request attention computation is still reading it on the compute stream. No event bridges the two streams, so a write-after-read (WAR) hazard window opens.

Fix: Record a compute-done event once (shared between load and store) and have both paths wait_event on it before launching DMA. This is the symmetric barrier the store path already had, applied to loads too.

Verified by the issue author: garble rate 4.3% baseline → 0% with fix (Fisher exact p≈5e-41). No measurable throughput cost in healthy regimes.

Changes:

• Rename _store_compute_done → _compute_done (shared event)
• Record once before both load and store launches
• Pass wait_event=self._compute_done to load path as well
• Updated docstring to reflect both paths are synced

[claude]

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @zaid646.

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