Adding cpp example - 2D imcompressible cylinder flow

[yiyuef]

Description

This PR adds a new C++ example cylinder_flow_2D that solves 2D incompressible channel flow past a cylinder-like obstacle (implemented as a masked no-slip region) using a projection (pressure-correction) method built on MOLE mimetic operators.

It also adds Sphinx/MyST documentation for the example and includes a representative output figure.

Type of Change

• New core functionality
• New example
• Documentation update
• Bug fix
• Performance improvement

Mathematical Details

Not applicable (no new operators added).
The example solves the incompressible Navier–Stokes equations with a fractional-step (projection) method and uses MOLE discrete operators (divergence, gradient, Laplacian) for the Helmholtz/Poisson solves.

Testing

• Unit tests pass
• Examples run successfully
• Convergence studies completed (if applicable)
• Cross-platform compatibility verified

Documentation

• Code is well-commented
• API documentation updated
• Example documentation added (if applicable)
• Mathematical background provided

Related Issues

#211

Additional Notes

• Output image cylinder_flow_2D_output1.png is included alongside the documentation page.