Use MSVC intrinsics for full multiplication

[commit-msuyog]

full_multiplication() currently uses unsigned __int128 on GCC/Clang and falls back to four 32-bit partial products everywhere else. This adds the native intrinsics for the two MSVC targets that have them.

• x64: _umul128(a, b, &high) gives both halves of the 128-bit product.
• ARM64: a * b provides the low half and __umulh(a, b) provides the high half.
• <intrin.h> is included only for MSVC x64/ARM64 builds, so other compilers and platforms are unaffected.
• The __SIZEOF_INT128__ branch remains first. This is important for clang-cl, which defines that macro and should continue using unsigned __int128.
• 32-bit MSVC and other compilers continue to use the existing portable fallback unchanged.

I extended the existing "128-bit products and leading zeros" test section with fixed edge cases checked against the existing big-integer reference, including 0, 1, UINT64_MAX, values around 2^32, and large boundary values. The existing deterministic random-pair coverage is also preserved.

single_include/nlohmann/json.hpp is synchronized with the implementation change.

Testing: the touched tests pass locally with ASan and UBSan, and git diff --check is clean. MSVC/clang-cl are not available in my local environment, so the new MSVC branches have not been compiled locally. The repository's Windows CI jobs should provide compiler-specific validation.

Performance: I have not benchmarked this change. The goal is to use the native 64×64-bit multiplication support provided by MSVC rather than the portable partial-product implementation.

Closes #5780

[github-actions]

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[nlohmann]

Please rebase to the latest develop: CI on develop is green again since #5779, so a rebase should give this PR a clean CI run.

(Comment written by Claude Code on behalf of @nlohmann.)

[nlohmann]

Thanks for the PR! The intrinsics are used correctly, and the branch order keeps clang-cl/icx on unsigned __int128. The msvc (x64) and msvc-arm64 jobs build with /W4 /WX and run ctest, so both new branches have been compiled and checked against the big-integer reference in CI.

A few things need to be done before this can be merged:

1. DCO: The commit has no Signed-off-by line. Please sign off your commits (git commit -s, or git rebase --signoff develop for existing commits) and force-push. See the DCO check for details.
2. Formatting: The check job fails because astyle formats the edge_cases initializer in tests/src/unit-class_lexer.cpp differently. Running make amalgamate fixes it (it runs astyle as well).
3. Merge develop: The other failures (-Wsuggest-attribute=pure with GCC, -Wexit-time-destructors in unit-diagnostics-optimized.cpp with Clang, Infer, and the cancelled jobs) do not come from this PR. The branch is based on a commit before #5779, which fixed them. Merging develop should make CI green.

See the inline comments for smaller points.

On performance: as noted in #5780, the gain on develop is small because the only caller is the Eisel-Lemire fallback. The measurement on MSVC x64 (dump() of canada.json) makes more sense once the zero-copy view stack (#5768) has landed, so no benchmark is needed for this PR.

---

This review was written by Claude Code.

[nlohmann]

The branch is still based on 30e542e, so the GCC, Clang and Infer failures already fixed on develop by #5779 will probably come back. Those failures aren't caused by this PR. The new commits only touch bit_ops.hpp and the test, which don't affect them.

[commit-msuyog]

Rebased onto the latest develop including #5779.

[nlohmann]

Looks good to me.

[nlohmann]

Thanks! Now let's wait for the Windows jobs and AppVeyor to pick this up.

[github-actions]

🔴 Amalgamation check failed! 🔴

The source code has not been amalgamated and/or formatted correctly, or BUILD.bazel is out of date.

📎 A ready-to-apply patch is attached to the failed workflow run as the amalgamation-patch artifact. Download it, then apply it locally from the repository root with:

git apply amalgamation.patch

This does not require installing astyle yourself.

@commit-msuyog Please read and follow the Contribution Guidelines.

[github-actions]

🔴 Amalgamation check failed! 🔴

The source code has not been amalgamated and/or formatted correctly, or BUILD.bazel is out of date.

📎 A ready-to-apply patch is attached to the failed workflow run as the amalgamation-patch artifact. Download it, then apply it locally from the repository root with:

git apply amalgamation.patch

This does not require installing astyle yourself.

@commit-msuyog Please read and follow the Contribution Guidelines.