Add fixed-size 6x6 dense LU factorization with benchmark.

mju_factorLU6/mju_solveLU6: same algorithm as mju_factorLU/mju_solveLU
with compile-time size, allowing full unrolling. At n=6, factor+solve is
25% faster than the runtime-sized version (93 vs 124 ns), and fixed-size
LU factorization is faster than generic dense Cholesky (55 vs 61 ns):
at this size, runtime-n loop overhead outweighs Cholesky's 2x flop
advantage. See new lu_benchmark_test. Results agree with the generic
version to rounding, not bitwise: the compiler may fuse (FMA) the
unrolled version differently.

Also add two DenseLU tests: a pivoting-required matrix with zero
diagonal, and fixed-vs-generic agreement.

PiperOrigin-RevId: 947705056
Change-Id: I24c54c9510964aa376886e9dd721890eda9889d3

test/benchmark/CMakeLists.txt

@@ -67,6 +67,12 @@ mujoco_test(
   ADDITIONAL_LINK_LIBRARIES benchmark::benchmark absl::core_headers
 )
 
+mujoco_test(
+  lu_benchmark_test
+  MAIN_TARGET benchmark::benchmark_main
+  ADDITIONAL_LINK_LIBRARIES benchmark::benchmark absl::core_headers
+)
+
 mujoco_test(
   crb_benchmark_test
   MAIN_TARGET benchmark::benchmark_main