Fix mldivide (backslash) to solve linear systems instead of computing inverse

[bardo84]

Category

Common script semantics and runtime correctness

Problem

mldivide / backslash currently returns a matrix inverse instead of solving the corresponding linear system.

Intent

This is a core MATLAB numerical semantics issue, not an edge case. If backslash is wrong, ordinary linear algebra code is not trustworthy.

Done When

• A \ b behaves like MATLAB's linear solve semantics for supported inputs
• the implementation does not substitute explicit matrix inversion where a solve is expected
• representative regression coverage exists for common valid inputs