Limb move and byte serialization APIs

[mmurshed]

Description

This PR adds native binary import/export and zero-copy limb manipulation APIs to BigInteger to avoid the overhead of decimal string parsing (Parse) and formatting (ToString).

Changed/New APIs

1. Zero-Copy Limb APIs:

   • BigInteger(std::vector<DataT>&& aInt, bool negative): Move constructor for raw limb vectors.
   • std::vector<DataT> ReleaseInteger(): Moves the internal limb vector out, leaving the object in a canonical zero state.

2. Byte Serialization APIs:

   • std::vector<uint8_t> ToByteArray(bool bigEndian = true) const: Serializes the magnitude to a byte array (MSB-first by default).
   • static BigInteger FromByteArray(std::span<const uint8_t> bytes, bool negative, bool bigEndian = true): Reconstructs a BigInteger from magnitude bytes and a sign flag.

Tests Run

We added unit tests to test_construction.cpp covering all new functionality, and verified with ctest:

cmake -S . -B build -DCMAKE_BUILD_TYPE=Release
cmake --build build -j8
cd build && ctest --output-on-failure

All test suites passed:

• mult_correctness
• div_correctness
• unit_tests
• mfa_roundtrip