Add LSDynaUMATTarget backend #4

Goal

Generate an LS-DYNA user material subroutine. LS-DYNA uses Fortran-callable umatNN entry points (one per material model number, 41–50 are user-defined slots) with yet-another convention.

LS-DYNA UMAT signature (umat41 / umat42 / ... / umat50)

Per the LS-DYNA User-Defined Materials manual:

extern "C" void umat41_(
    double *cm,     // (lmc)     material constants
    double *eps,    // (nlq, 6)  strain increment, block-major
    double *sig,    // (nlq, 6)  stress, in/out
    double *epsp,   // effective plastic strain (scalar) [for some models]
    double *hsv,    // (nlq, nhv) history variables
    double *dt1siz, // (nlq)     timestep size
    double *capa,   // (nlq, ...) auxiliary
    char   *etype,  // element type tag
    double *tt,     // current time
    double *temps,  // temperatures
    int    *failel, // failure flag
    double *cma,    // additional material constants
    int    *qmat);  // material model variant

Block-major layout like Abaqus VUMAT, but different convention details:

Notable

LS-DYNA is explicit-only by default for user materials (no tangent required). The codegen can skip OutputRole::ConsistentTangent for this target — same simplification as VUMAT.

LS-DYNA component order matches ANSYS — so the Voigt machinery from #3 carries over.

What to emit

• targets/lsdyna_umat.h with LSDynaUMATTarget.
• extern \"C\" entry point named after the user-material slot number (configurable via target constructor: LSDynaUMATTarget(slot=42)).
• Block-major iteration like VUMAT (Add AbaqusVUMATTarget backend (vectorized UMAT) #2).

Acceptance criteria

• Smoke test against LinearElasticShear.
• Test that emits with configurable slot number (e.g. umat41_ vs umat45_).
• Voigt machinery shared with ANSYS path.

Dependencies

• Add AbaqusUMATTarget backend #1 first (general Voigt boundary).
• Add AnsysUSERMATTarget backend #3 (ANSYS) shares the component-order convention; either could precede.