Prepare 4.2.1 release

[ProfFan]

This prepares the 4.2.1 patch release branch on top of release/4.2.

Included here:

• bump release metadata to 4.2.1
• adapt Boost detection for current/Homebrew Boost while keeping Boost.Filesystem linked and the Boost floor at 1.65
• fix the missing Boost.Range include in testLinearEquality so check passes locally
• backport the production wheel build workflow for the 4.2 line
• pass the CMake policy floor explicitly in wheel builds
• build Boost.Filesystem in the wheel bootstrap
• split macOS wheel builds into separate arm64 and x86_64 jobs instead of labeling them universal2
• pin NumPy below 2.0.0 for wheel compatibility with the vendored pybind11 stack

Local validation:

• full check target passes in the 4.2.1 worktree

Follow-up after merge:

• tag 4.2.1 on release/4.2
• merge release/4.2 forward into develop

[dellaert]

Yay!!

[dellaert]

I downloaded the artifacts and tested 3.11 and 3.14 on silicon Apple silicon. Everything worked fine.
I uploaded all wheels to PyPy. https://pypi.org/project/gtsam/4.2.1/

[ProfFan]

Thanks Frank! We should add an action to auto-promote these to primary PyPI in the future :)