Add matrix-free action assembly (stiffness_action / mass_action)

[lxmota]

Summary

• Adds assemble_matrix_free_action! — a new assembly path that accumulates K·v (or M·v) directly as a vector per element, without forming the full element stiffness/mass matrix
• Adds abstract function stubs stiffness_action and mass_action to Physics.jl for physics implementations to dispatch on
• Exports assemble_matrix_free_action!, stiffness_action, and mass_action

Motivation

The existing assemble_matrix_action! path accumulates a full SMatrix{24,24} element stiffness matrix (576 Float64 = 4.6 KB) per GPU thread before multiplying by v_el. This far exceeds the GPU VGPR budget (~256–512 registers), causing register spilling to global memory and ~570 ms/call on an RX 7600 for a 530k-DOF torsion problem in Carina.jl.

The new matrix-free path accumulates Kv_el = SVector{24} directly by calling func_action(physics, ..., v_el, ...) per quadrature point, keeping only ~42 doubles of intermediates. Benchmark results on RX 7600 (530k DOFs):

Operation	Old (assemble_matrix_action!)	New (assemble_matrix_free_action!)	Speedup
Stiffness matvec	~570 ms	~64 ms	~9×
Mass matvec	~124 ms	~4 ms	~31×
Combined (K + c_M·M)·v	~68 ms	~10×

Interface

Physics implementations define:

FEC.stiffness_action(physics, interps, x_el, t, dt, u_el, u_el_old, v_el,
                     state_old_q, state_new_q, props_el) → SVector
FEC.mass_action(physics, interps, x_el, t, dt, u_el, u_el_old, v_el,
                state_old_q, state_new_q, props_el) → SVector

Assembly call:

FEC.assemble_matrix_free_action!(assembler, FEC.stiffness_action, Uu, Vu, p)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 62.43%. Comparing base (4de17f2) to head (71365ed).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #242      +/-   ##
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+ Coverage   61.90%   62.43%   +0.53%     
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  Files          60       60              
  Lines        3344     3381      +37     
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+ Hits         2070     2111      +41     
+ Misses       1274     1270       -4     

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