[CK] Fix 32-bit overflow in batch prefill kernel for >4GB KV
cache (#4999)
Use SRD rebasing for page_block_size >= kN0: move SRD base pointer to
page start via 48-bit arithmetic, encode only within-page offset in
voffset. Original code path preserved for ps1/ps16 via constexpr-if.
## Motivation
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## Technical Details
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## Test Plan
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