As far as I can tell, this is the only Pakistan-tax payroll engine shipped cleanly anywhere. FBR slabs change per fiscal year and per regime; EOBI rates are separate; consolidated-versus-component salary structures route through different math; bonuses entered late roll forward two months per §17.6. All of that lives in one pure module, with seventeen vitest property tests covering the invariants (1000-iteration round-trip, EXEMPT regime, bonusTaxedAsSalary=false, GROSS_AGREED bonus invariance).
The Net→Gross solver uses bisection rather than Newton damping, after a discovered counterexample where a CONSOLIDATED-structure salary with a bonus-induced slab transition broke Newton's derivative continuity assumption and diverged by ~10k PKR. Bisection is slower; bisection is correct. The doc-comment in the engine records the counterexample so the next person doesn't "optimize" it back.