E1: Flattening benchmark example

[csparker247]

Summary

Add a `BenchmarkFlattening` example app that measures and compares wall-clock runtime for multiple flattening configurations across mesh sizes. Motivated by volume-cartographer#123 which benchmarked ABF++ + LSCM (SparseLU/CG) but noted HLSCM as a future alternative.

Desired Output

Markdown table printed to stdout, e.g.:

Num. faces	ABF+LSCM (SparseLU)	ABF+LSCM (LSCG)	ABF+HLSCM (LSCG)
...	...	...	...

Acceptance Criteria

• New openabf_example_benchmark target added to examples
• Accepts one or more mesh files as CLI arguments
• Times ABF++, then separately times each LSCM variant (SparseLU, LSCG, HLSCM)
• Prints results as a markdown table (faces, ABF time, LSCM-SparseLU, LSCM-LSCG, HLSCM)
• Works with any OBJ/PLY mesh

Related

• Downstream: ABF: Add support for other solvers volume-cartographer#123
• F1 (HLSCM implementation): HLSCM (Hierarchical LSCM) #5

[csparker247]

Track E1 (Flattening benchmark example) has been completed. Closing.