Try nanobind 3 split mode

[jdumas]

Description

Try nanobind 3's new split mode using the 3.0.0.dev3 development release:

• build lagrange_python with BACKEND_MODULE nanobind_backend
• pin nanobind-backend==1.0.0.dev3 while development releases have version-specific platform ABIs
• emit a single cp310-abi3 wheel per platform
• restrict cibuildwheel to the cp310-* build selector
• update custom type casters and limited-API-incompatible calls for nanobind 3

References:

• https://nanobind.readthedocs.io/en/latest/split_mode.html
• https://nanobind.readthedocs.io/en/latest/changelog.html

Related Issue

N/A — this is an experimental PR to exercise nanobind 3 split mode and its wheel CI.

Motivation and Context

Split mode separates nanobind's stable-ABI frontend from its version-specific backend. This should reduce Lagrange's wheel matrix to one wheel per platform while retaining a version-specific nanobind backend.

How Has This Been Tested?

• Built a wheel locally with uv build --wheel; it was tagged cp310-abi3 and contained lagrange/lagrange.abi3.so.
• Installed and imported that same wheel on Python 3.10 and Python 3.14 using nanobind-backend==1.0.0.dev3.
• Ran the Python test suite on Python 3.14: 479 passed, 4 skipped.
• Ran actionlint on .github/workflows/wheel.yaml.
• Ran clang-format --dry-run --Werror on the changed C++ files.
• Ran git diff --check.

Screenshots (if appropriate):

N/A

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.