Add BigInt::signum

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[Copilot]

Pull request overview

This PR introduces a new BigInt::signum() API and migrates a broad set of call sites across the library, tests, and fuzzers from the legacy is_positive()/is_negative()/is_nonzero() predicates to signum()-based checks. This helps make sign handling less ambiguous (notably around is_positive() historically including zero) and centralizes sign/zero interpretation.

Changes:

• Add BigInt::signum() and deprecate is_nonzero(), is_negative(), and is_positive() in favor of signum() comparisons.
• Update numerous math/number theory, EC, RSA, X.509, FFI, codec, tests, and fuzzers to use signum() and is_zero().
• Tighten/clarify several argument checks and assertions by switching to explicit signum comparisons.

Reviewed changes

Copilot reviewed 25 out of 25 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/lib/math/bigint/bigint.h	Adds signum(), updates operator!, deprecates sign helpers in favor of signum()
src/lib/math/bigint/bigint.cpp	Replaces sign checks in comparisons/conversions/reductions with signum()
src/lib/math/bigint/divide.cpp	Uses signum() for divisor/operand sign checks and nonzero checks
src/lib/math/bigint/big_ops2.cpp	Migrates modular ops and shifts to signum()-based sign/zero handling
src/lib/math/bigint/big_ops3.cpp	Migrates division/modulo/shift sign checks to signum()
src/lib/math/bigint/big_code.cpp	Uses signum() for string encoding sign handling and positivity checks
src/lib/math/bigint/big_rand.cpp	Uses signum() in random integer range validation
src/lib/math/numbertheory/barrett.cpp	Updates modulus/argument validation and CT precheck positivity to signum()
src/lib/math/numbertheory/primality.cpp	Uses signum() for argument validation and sign flips
src/lib/math/numbertheory/numthry.cpp	Uses signum() for modulus negativity checks in power_mod
src/lib/math/numbertheory/monty_exp.cpp	Uses signum() to validate multi-exponentiation exponents
src/lib/math/numbertheory/mod_inv.cpp	Refactors inverse-mod argument validation and internal nonzero/negative checks to signum()
src/lib/math/numbertheory/make_prm.cpp	Migrates coprime validation to signum() checks
src/lib/pubkey/rsa/rsa.cpp	Uses signum() for RSA parameter validation
src/lib/pubkey/ec_group/ec_group.cpp	Uses signum() for EC parameter validation and clarifies A/B/order constraints
src/lib/pubkey/ec_group/ec_apoint.cpp	Uses signum() to validate affine coordinates are non-negative
src/lib/pubkey/ec_group/legacy_ec_point/point_mul.cpp	Uses signum() for scalar sign validation and sign comparison
src/lib/pubkey/ec_group/legacy_ec_point/ec_point.cpp	Uses signum() for scalar sign in mul; updates internal assert to signum()
src/lib/x509/x509cert.cpp	Uses signum() to record serial sign
src/lib/asn1/ber_dec.cpp	Uses signum() to reject negative decoded integers
src/lib/ffi/ffi_mp.cpp	Updates FFI sign queries to use signum()
src/lib/codec/base58/base58.cpp	Switches loop condition to !is_zero()
src/tests/test_bigint.cpp	Updates test preconditions to use signum() for sign checks
src/fuzzer/invert.cpp	Uses !is_zero() and signum() for loop and sign normalization
src/fuzzer/bn_cmp.cpp	Uses signum() assertions instead of deprecated sign/zero helpers

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