Accept vector-valued per-qp kernels in the diagonal assembler

[lxmota]

Summary

Adds one method to the diagonal assembly path: _accumulate_q_value(::AssembledDiagonal, ...) now also accepts a per-quadrature-point SVector, so a physics-side kernel can return the element diagonal directly instead of a full NxN element matrix that the assembler immediately reduces to its diagonal. The existing SMatrix method is untouched; the two dispatch on the kernel's return type.

Motivation

Carina's matrix-free Jacobi/Chebyshev/AMG preconditioner updates were computing every entry of the 24x24 element stiffness per quadrature point and keeping 24 of them. A CUPTI profile of a production Newmark step measured that kernel at 585 ms per Newton iteration on a V100 — ~100x the per-element cost of the matrix-free action (see Carina benchmark/crosscode/README.md §4). With this method, Carina provides diagonal-only kernels (diag(JxW·G·A·G') needs only the tangent entries pairing a component with itself): the preconditioner update drops 587 → 12.4 ms on a V100 (47x) and 290 → 6.5 ms on an A100 (45x), contributing to end-to-end per-step gains up to 1.9x.

Testing

• Reviewed on CPU and GPU per the usual protocol:
  • CPU: full Carina test suite passes, including new testsets pinning the vector-returning diagonal kernels to the full-matrix diagonals (rtol 1e-13) at a deformed state, plus the LinearElastic tangent-at-zero specialization.
  • GPU: end-to-end torsion benchmarks on RX 7600 (ROCm), V100 and A100 (CUDA) hold all correctness invariants (4,455 CG iterations, |U|_max = 3.98e-02) with the diagonal path in use every Newton iteration.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.99%. Comparing base (beda361) to head (76ad13e).

Files with missing lines	Patch %	Lines
src/assemblers/Assemblers.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #337      +/-   ##
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- Coverage   69.01%   68.99%   -0.03%     
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  Files          55       55              
  Lines        6523     6525       +2     
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  Hits         4502     4502              
- Misses       2021     2023       +2     

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