Generalize Mat×Vec operator* to support mixed scalar types

[Copilot]

The matrix-vector operator* was overly generic, accepting any Vector type, which obscured intent and prevented mixed-type scalar multiplication.

Changes

• Mat.hpp: Replace unconstrained Vector template parameter with explicit Vec<T2, N>, returning Vec<T1, N> (matrix scalar type). Enables multiplying a Mat<M,N,T1> by a Vec<T2,N> with differing scalar types.

  // Now supported:
  Mat<4, 4, double> m{...};
  Vec<float, 4> v{...};
  auto result = m * v;  // Vec<double, 4>

• Mat.hpp: Remove spurious trailing semicolon from default constructor.
• String.hpp: Take structured binding by const& in split() range loop to avoid unnecessary copies.

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