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fix(bigint): correct to_uint/to_uint64 truncation of large negatives

The non-js to_uint/to_uint64 (and hence to_int/to_int64) computed the
truncation of a negative value by adding one modulus and reading
magnitude limbs. That is wrong whenever the sum is still negative,
i.e. whenever |x| > 2^32 (resp. 2^64) — already -(2^32 + 1) was
mishandled: (-(1N << 32) - 1N).to_uint() returned 1 instead of
4294967295, and (-(1N << 33) - 5N).to_uint() returned 5 instead of
4294967291; only exact modulus multiples and scattered coincidental
residues came out right. js was already correct via BigInt.asUintN and
is unchanged.

Truncation to 2^k is a ring homomorphism, so for a negative value it is
the wrapping negation of the magnitude's low bits; compute it that way.

Fixes #4050

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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