Integrate diff-engine as git submodule

[Transurgeon]

Make diff-engine a pure C library with CVXPY handling all Python integration via git submodule. Single pip install . builds everything.

Changes to diff_engine_core (submodule):

• Remove pyproject.toml (Python packaging now handled by CVXPY)
• Remove src/dnlp_diff_engine/ Python wrapper
• Add Apache 2.0 LICENSE file
• Rename module from _core to _diffengine
• Add diag_vec atom implementation

Changes to CVXPY:

• Add diff_engine_core as git submodule
• Add diffengine Extension in setup/extensions.py
• Update setup.py to include diffengine in extensions list
• Update imports from dnlp_diff_engine to _diffengine
• Add submodules: recursive to CI workflows

Description

Please include a short summary of the change.
Issue link (if applicable):

Type of change

• New feature (backwards compatible)
• New feature (breaking API changes)
• Bug fix
• Other (Documentation, CI, ...)

Contribution checklist

• Add our license to new files.
• Check that your code adheres to our coding style.
• Write unittests.
• Run the unittests and check that they’re passing.
• Run the benchmarks to make sure your change doesn’t introduce a regression.