Add isolated Commons Math CLAS12 swimmer

[heddle]

Summary

• add the experimental cnuphys:clas12-swimmer Maven artifact
• keep the existing production cnuphys:swimmer sources and behavior untouched
• implement the CLAS12 swimming API with Apache Commons Math ODE solvers
• include characterization tests for straight-line, fixed-step, plane, sphere, z-line, and beamline swimming

Isolation

The new code lives under cnuphys.CLAS12Swim in its own Maven module. Production coatjava does not reference or select it. The artifact layers on the existing trajectory and magnetic-field APIs so downstream applications can evaluate it without replacing the production swimmer.

Verification

• mvn -pl common-tools/cnuphys/clas12-swimmer -am clean verify
• 8 experimental swimmer tests pass
• downstream CED clean Linux Java 21 CI passes with the experimental artifact and production swimmer loaded side by side
• manual CED smoke tests cover particle swimming, event-file swims, 2D drawing, and lazy-created 3D views

This PR is intentionally a draft while the experimental swimmer is evaluated.