Release 0.8.6

SOLVAX 0.8.6 makes the tridiagonal solvers complex-capable without giving up the fast real paths:

• tridiagonal_solve and cyclic_tridiagonal_solve accept complex operands. Real bands with a complex right-hand side solve the real and imaginary parts independently, keeping real band storage and the fused accelerator kernel; genuinely complex bands use the portable Thomas path, which is complex-safe on every supported JAX release. See Batched tridiagonal solve.

• The fused "lax" backend is wrapped in an implicit linear solve, so it is now forward- and reverse-differentiable even on JAX versions whose primitive defines no differentiation rule. Gradients through method="lax" and method="auto" match the Thomas path.

• The cyclic Sherman–Morrison derivation is stated with the algebraic transpose, matching the implementation for complex corner couplings.