viable/strict/1791693334: Let fastAtomicAdd derive its bound from a packed accessor (#196155)

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fastAtomicAdd needs a bound on how far it may look for a pairing partner when packing a 16-bit atomic. Callers holding a raw pointer supply that themselves, and getting it wrong is silent (too large lets the pairing write past the allocation) -- which is why ~20 scatter sites that subscript a packed accessor still use the plain atomic rather than hand-deriving a bound. Adds an accessor overload that derives offset a…

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