v1.6.1 — HinfSyn/H2Syn direct-feedthrough support

• HinfSyn: central controller was missing the γ⁻²·D21·B1ᵀ·X observer term; for plants with B1·D21ᵀ ≠ 0 (all mixed-sensitivity plants) the closed loop exceeded the reported γ. Controllers for those plants change; others are bitwise identical.
• HinfSyn and H2Syn handle D22 ≠ 0 via loop shift (K = K0(I + D22·K0)⁻¹); closed loop, γ and H2 norm unchanged; always well-posed.
• ErrH2DirectFeedthrough is deprecated; H2Syn no longer returns it.