trunk/e97ee94e32f26ccec33db4c1411bddc4cc9e1a31: Let fastAtomicAdd derive its bound from a packed accessor (#196155)

Executive Summary

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 and bound itself: offset = sum over d of index[d] * stride[d] span = 1 + sum over d of (size[d] - 1) * stride[d] span is the distance to the last addressable element, not the element count -- the two differ for a non-contiguous accessor, whose gaps are legal...

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