Packed two-limb Posit signed sums #
Packed two-limb signed sums align decoded dyadic fields, combine their signed magnitudes, and
round within UInt128 whenever the exact intermediate fits that carrier. The sole exact
capacity branch is selected by native shift and carry checks; it preserves the same general
dyadic semantics for intermediates that genuinely require more than 128 bits.
Exact capacity branch for a signed sum whose aligned magnitude does not fit in two limbs.
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Round two nonzero signed magnitudes already aligned at one exponent.
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Round the exact sum of two decoded native dyadics.
Zero operands bypass alignment. A nonzero operand with the larger exponent is shifted to the smaller exponent when its exact value remains in two limbs. Failed capacity checks enter the one exact wider-intermediate branch.
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Add two packed Posit words and retain the complete result in two native limbs.
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Subtract two packed Posit words by reversing the right decoded sign.