Fix missing quasisymmetry re-export causing "Test conda env" job failure

[Copilot]

The "Test conda env" job (run 34170619332) failed with AttributeError: module 'vmecpp.simsopt_compat' has no attribute 'magnetic_field_strength' (and two similar errors), because tests/test_qs.py calls these functions on vmecpp.simsopt_compat but they were never actually exposed there.

Root cause

• src/vmecpp/_qs.py defines magnetic_field_strength, quasisymmetry_residuals, and quasisymmetry_total.
• Its docstring states these are "Re-exported from vmecpp.simsopt_compat", but the rename commit that introduced this comment never added the corresponding import — the re-export was a no-op comment.

Fix

• Added the missing import in src/vmecpp/simsopt_compat.py to re-export magnetic_field_strength, quasisymmetry_residuals, and quasisymmetry_total from vmecpp._qs.

from vmecpp._qs import (  # noqa: F401
    magnetic_field_strength,
    quasisymmetry_residuals,
    quasisymmetry_total,
)

• Included src/vmecpp/_qs.py and tests/test_qs.py, which were part of the same in-flight change but not yet present on this branch, so the re-export has something to target and the fix is exercised by the existing test.

[github-actions]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 1.50.

Benchmark suite	Current: 740cf56	Previous: de5166b	Ratio
benchmarks/test_benchmarks.py::test_bench_simsopt_finite_difference_gradient	0.4870138609999799 seconds (stddev: 0)	0.3059909609996794 seconds (stddev: 0)	1.59

This comment was automatically generated by workflow using github-action-benchmark.