Add AbaqusUMATTarget backend #1

Goal

Generate an Abaqus UMAT user subroutine (Fortran-callable C++ via extern "C") from a ConstitutiveModel recipe. This is the second target after MOOSE; doing it forces the Voigt machinery that ANSYS and LS-DYNA will reuse.

Abaqus UMAT signature

The UMAT entry point (Fortran calling convention, C++-side declared via extern "C"):

extern "C" void umat_(
    double *STRESS,        // (NTENS)        in/out: Cauchy stress, Voigt
    double *STATEV,        // (NSTATV)       in/out: state variables, flat array
    double *DDSDDE,        // (NTENS, NTENS) out:   Jacobian dσ/dε, Voigt 6×6
    double *SSE, double *SPD, double *SCD,
    double *RPL, double *DDSDDT, double *DRPLDE, double *DRPLDT,
    double *STRAN,         // (NTENS)        in:    strain at start of step, Voigt
    double *DSTRAN,        // (NTENS)        in:    strain increment, Voigt
    double *TIME, double *DTIME, double *TEMP, double *DTEMP,
    double *PREDEF, double *DPRED,
    char   *CMNAME, int CMNAME_LEN,
    int    *NDI, int *NSHR, int *NTENS, int *NSTATV,
    double *PROPS,         // (NPROPS)       in:    material constants, positional
    int    *NPROPS,
    double *COORDS,
    double *DROT,
    double *PNEWDT,
    double *CELENT,
    double *DFGRD0, double *DFGRD1,
    int    *NOEL, int *NPT, int *LAYER, int *KSPT,
    int    *KSTEP, int *KINC);

For 3D: NTENS = 6, Voigt component order {11, 22, 33, 12, 13, 23} (per Abaqus convention). For shear strains in STRAN/DSTRAN the engineering convention is used (γ_ij = 2 ε_ij off-diagonals).

Recipe mapping

What needs to be emitted

A single .cpp file containing:

1. The embedded Layer-2 compute function (anonymous namespace).
2. The extern "C" UMAT entry point with the exact Abaqus signature.
3. Boundary code that:
   • Constructs tmech::adaptor<..., abq_std<3, true>> over STRAN / DSTRAN / STRESS.
   • Unpacks each history variable from STATEV by sequential offset.
   • Unpacks each parameter from PROPS by declaration order.
   • Calls the Layer-2 compute function.
   • Writes outputs back via the corresponding adapters.

Acceptance criteria

• New targets/abaqus_umat.h with AbaqusUMATTarget class.
• Smoke test: emits the LinearElasticShear example as a UMAT, string-checks the extern "C" signature and the Voigt adapter calls.
• Documentation: README target table updated.

Dependencies

• tmech's abq_std adapter (already available).
• No numsim-cas changes required.

Estimated effort

1-2 days including tests.