stillwater-sc / universal / 26374509558

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fix(ereal): renormalize expansion_product output to Priest canonical form (#981) (#982)

expansion_product accumulated partial products via linear_expansion_sum but
never renormalized the final result, so ereal multiplication (and division,
which is e * reciprocal) returned non-canonical expansions: overlapping
components, and limb counts far past the value's true non-overlapping length
(an ereal<16> product grew to ~69 limbs, ~93% with overlapping components).
operator+= / operator-= were already correct (they renormalize).

Add a closing renormalize_expansion() to expansion_product (guarding the zero
case). renormalize_expansion is exactly value-preserving (Priest canonical
sweepUp/recurse/sweepDown), so this changes the limb structure but not the
value.

Verified over 20,000 random ereal<16> pairs: multiplication and division
non-overlap violations drop from ~18.6k to 0, limb counts from ~69 to ~21
(the true non-overlapping length of the product's value), and the projected
value is unchanged in every case. This is the mul/div analogue of #959, which
fixed non-canonical output for + by adopting the canonical renormalize.

Full ereal regression set passes on gcc-13 and clang-18: addition,
subtraction, multiplication, division, algebraic_identities, and the
exponent/logarithm/sqrt/pow/trigonometry mathlib tests (which exercise mul/div
heavily) -- no regressions, and the values are now Priest-canonical.

Resolves #981
Relates to #944
Relates to #954

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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