3.1.3

• Z-factor Z=Z-B reformulation*: BNS Peng-Robinson cubic solver now uses Aaron Zick's Z* = Z - B variable substitution, which bounds all physical roots to (0, 1] and guarantees Halley solver convergence. Eliminates rare failures at extreme pressures. Falls back to Cardano analytically if needed.
• Rust batch vectorization: Rust-accelerated gas_z() and gas_ug() now use batch dispatch (single FFI call for all pressures) instead of per-pressure scalar loops. Precomputes critical properties, BIPs, and LBC mixture parameters once per call. New Rust functions: bns_zfactor_batch, dak_zfactor_batch, hy_zfactor_batch, gas_ug_lge_batch, gas_ug_lbc_batch. LBC viscosity batch is 9.3x faster than scalar; full BNS pipeline (Z + viscosity) is 2x faster than pure Python.
• Rust GWR inverse Laplace transform: Rust/MPFR-accelerated Gaver-Wynn-Rho algorithm and Bessel functions for influence table generation. Switched dependency from gwr_inversion to ilt-inversion.
• oil_co() Pb discontinuity fix: Fixed oil compressibility discontinuity at bubble point in black oil tables. Added undersaturated_only mode.
• 630 validation tests (up from 603 in 3.0.5).