Add AnsysUSERMATTarget backend #3

Goal

Generate an ANSYS USERMAT user subroutine. ANSYS Mechanical's user-defined material framework — Fortran-callable, similar shape to Abaqus UMAT but with different component conventions and argument ordering.

USERMAT signature (simplified)

extern "C" void usermat_(
    int    *matId, int *elemId, int *kDomIntPt, int *kLayer, int *kSectPt,
    int    *ldstep, int *isubst, int *keycut,
    int    *nDirect, int *nShear, int *ncomp,
    int    *nStatev, int *nProp,
    double *Time, double *dTime,
    double *Temp, double *dTemp,
    double *stress,        // (ncomp)        in/out
    double *ustatev,       // (nStatev)      in/out
    double *dsdePl,        // (ncomp, ncomp) out: Jacobian
    double *sedEl, double *sedPl, double *epseq,
    double *Strain,        // (ncomp)        in
    double *dStrain,       // (ncomp)        in
    double *epsPl,         // (ncomp)        in/out
    double *prop,          // (nProp)
    double *coords,
    double *rotateM,
    double *defGrad_t, double *defGrad,
    double *tsstif, double *epsZZ,
    double *var0, double *var1, double *var2, double *var3, double *var4,
    double *var5, double *var6, double *var7, double *var8);

Convention differences from Abaqus

The component-order swap means we need a different tmech Voigt adapter than abq_std. Either:

• Extend tmech with an ansys_std adapter, or
• Use abq_std and add a 4×4 permutation matrix at the boundary.

Verify with tmech maintainer (you) which is preferable.

Tangent layout

ANSYS dsdePl is column-major (Fortran storage). The codegen needs to write the tangent in column-major order, or pre-transpose before writing.

What to emit

A .cpp file with:

1. Embedded Layer-2 compute function.
2. extern \"C\" USERMAT entry point.
3. Boundary code with ANSYS-convention adapters and the column-major tangent write.

Acceptance criteria

• targets/ansys_usermat.h with AnsysUSERMATTarget.
• Tests: signature match, ANSYS-order Voigt adapters used, column-major tangent write.
• Documentation in README.

Dependencies

• Add AbaqusUMATTarget backend #1 (AbaqusUMATTarget) — most patterns are shared.
• Possibly a tmech update for an ansys_std adapter (or use a runtime permutation).