Give F# a way to solve a system of equations (#562)

[Rafael-SOWNet]

Addresses #562.

You can be the one to pick up this redesign; keeping open until a redesign arrives.

Here it is. The wrapper had solutions x expr for one equation and nothing for a system, so the only route was dropping into the C# API.

solveSystem ["x"; "y"] ["x + y - 3"; "x - y - 1"]   // [[2, 1]]

Which of the two shapes, and why

The issue offered:

// Version 1
val system : list 'a -> EquationSystem
val solveSystem : EquationSystem -> Matrix

// Version 2
val solveSystem : list Variable -> list 'a -> Matrix

Version 2, because it reads like the solutions x expr already in the wrapper — subject first, then the thing — and because Version 1 makes a caller learn an EquationSystem type before they can solve anything. equationSystem is exposed as well, so Version 1's use (build one, pass it around) is available without Version 1 being the only door.

Two decisions the tests pin

An entry may be written either way. x + y = 3 and x + y - 3 mean the same system. An equality is moved to one side rather than passed on as a node — and that is not cosmetic: handing an Equalsf to EquationSystem makes the solver try to invert an equality and throw NotSufficientlySupportedException. That is what my first version did, and the tests caught it.

No solution answers None. EquationSystem.Solve returns a null Matrix, and in F# a null is worth turning into an option at the boundary. "There is no solution" and "the solver gave up" arrive the same way here, and neither is a matrix.

Evidence

• F# wrapper tests 134 passed, 0 failed (130 existing + 4 new)
• the tests cover: a solved linear system with columns following the order of vars, the two spellings agreeing, an inconsistent system answering None, and building a system to pass around
• purely additive — no existing function changes, so no BREAKING-CHANGES.md entry

Not included

The issue is one of three [WIP] F#/plotting redesigns (#575, #558) marked by their author in 2022 and never advanced. This does not speak to those — it takes the one concrete API the thread actually specified.

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