Refactor simplex and Lazy simplex improvement

[nathanneike]

Types of changes

Refactor
Performance improvement
Tests

Motivation and context / Related issue

This refactors the network simplex storage used by lazy EMD so that lazy mode avoids materializing dense per-arc storage where possible.

The lazy path now uses explicit storage/accessor modes for costs, flows, endpoints, and arc states. It also returns sparse transport output instead of allocating a dense transport matrix internally. This reduces memory usage for lazy EMD while keeping dense and sparse EMD behavior unchanged.

How has this been tested (if it applies)

Rebuilt the C++/Cython extension and ran the whole test suite

PR checklist

• I have read the CONTRIBUTING document.
• The documentation is up-to-date with the changes I made (check build artifacts).
• All tests passed, and additional code has been covered with new tests.
• I have added the PR and Issue fix to the RELEASES.md file.