Kernel benchmarks

The benchmark target is a small regression smoke suite, not a portable performance claim. Run it with the pinned minimum toolchain and retain the machine, compiler, feature set, and workload alongside any comparison:

cargo +1.85.0 bench --locked --all-features --bench kernels

The baseline below was recorded on the local development host after the M15 kernel expansion. Times are total wall-clock times for the stated number of short repetitions and are intended only for same-host comparisons.

periodic geometry and bonds (128 runs): 131.375us
interaction resolution (128 runs): 21.083us
dense matrix construction (32 runs): 632.375us
matrix-vector action (16 runs): 4.583us
dense diagonalization (2 runs): 652.541us
exact expectation (32 runs): 7.125us
TDVP statistics (128 runs): 264.75us
TDVP solve (32 runs): 46.166us
SSE sweeps (4 short runs): 454.083us

The target covers geometry and interaction resolution, dense matrix construction and action, exact diagonalization and expectation, TDVP statistics and solve, and a short SSE sweep. A future performance gate should compare like-for-like workloads and report compiler, CPU, feature, and thread settings rather than treating these values as universal thresholds.