Data: Solver with Simplex LP, integer branch and bound and a nonlinear method (#326)

data.solver maximises, minimises or targets a value in an objective cell by
changing variable cells under <=, >=, =, int and bin constraints. Simplex LP
linearises the model through recalculation, checks linearity and solves it
with a two-phase simplex; integers use branch and bound. The GRG Nonlinear
choice runs an augmented-Lagrangian projected-gradient method. The model is
saved as hidden solver_* names; Solver Parameters and Solver Results dialogs.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/mod.rs

@@ -14,6 +14,7 @@ pub mod pivot;
 pub mod print;
 pub mod review;
 pub mod sheet;
+pub mod solver;
 pub mod spelling;
 pub mod view;
 pub mod whatif;
@@ -118,6 +119,7 @@ pub fn command_specs() -> &'static [CommandSpec] {
         v.extend(extra::specs());
         v.extend(draw::specs());
         v.extend(spelling::specs());
+        v.extend(solver::specs());
         v.extend(pivot::specs());
         v.extend(whatif::specs());
         v.extend(print::specs());