Add AbaqusVUMATTarget backend (vectorized UMAT) #2

Goal

Generate an Abaqus VUMAT user subroutine. VUMAT is the explicit-dynamics, vectorized counterpart to UMAT — it operates on blocks of integration points at once, which makes it the standard target for high-throughput explicit Abaqus models.

Depends on #1 (AbaqusUMATTarget) — most of the Voigt + Abaqus convention machinery is shared.

VUMAT signature

extern "C" void vumat_(
    int    *nblock,        // number of points in the block
    int    *ndir, int *nshr, int *nstatev, int *nfieldv, int *nprops,
    int    *lanneal,
    double *stepTime, double *totalTime, double *dt,
    char   *cmname, int cmname_len,
    double *coordMp,
    double *charLength,
    double *props,         // (nprops)
    double *density,       // (nblock)
    double *strainInc,     // (nblock, ndir+nshr)
    double *relSpinInc,    // (nblock, nshr)
    double *tempOld,
    double *stretchOld,    // (nblock, ndir+nshr)
    double *defgradOld,    // (nblock, ndir+2*nshr) — note: 9 components in 3D
    double *fieldOld,
    double *stressOld,     // (nblock, ndir+nshr) — stress at start of step
    double *stateOld,      // (nblock, nstatev)
    double *enerInternOld, double *enerInelasOld,
    double *tempNew, double *fieldNew,
    double *stressNew,     // out: (nblock, ndir+nshr)
    double *stateNew,      // out: (nblock, nstatev)
    double *enerInternNew, double *enerInelasNew);

Block-major layout: stressNew[i * NTENS + k] where i is the point index and k is the component.

Differences from UMAT

The big win for VUMAT: no consistent tangent to generate. The codegen only needs to emit stress + history evolution.

What to emit

A .cpp file with the extern \"C\" VUMAT entry and a loop over nblock points that:

1. For each point, build tmech adapters over the block-major slices.
2. Call the Layer-2 compute function.
3. Write the stress and state outputs back.

Acceptance criteria

• New targets/abaqus_vumat.h with AbaqusVUMATTarget.
• Smoke test against a recipe with OutputRole::Stress only (skip tangent since VUMAT doesn't use it).
• Test that backend rejects recipes with OutputRole::ConsistentTangent (or warns and drops it).

Dependencies

• Add AbaqusUMATTarget backend #1 must land first (shares the Voigt machinery).