Delegate *= scalar to blas.dscal

[Copilot]

The JVM Array[Double] vector ops were inconsistently using BLAS. dot, norm, +=, -=, and /= all delegate to BLAS, but *=(d: Double) was using a manual SIMD loop instead of dscal — the obvious Level 1 BLAS routine for in-place scalar multiplication.

Change

• *=(d: Double): Replace 12-line SIMD loop with blas.dscal(vec.length, d, vec, 1), matching the pattern already used by /=
• *(d: Double): benefits automatically since it delegates via vec.clone.tap(_ *= d)

// Before
inline def *=(d: Double): Unit =
  var i = 0
  while i < spd.loopBound(vec.length) do
    DoubleVector.fromArray(spd, vec, i).mul(DoubleVector.broadcast(spd, d)).intoArray(vec, i)
    i += spdl
  end while
  while i < vec.length do
    vec(i) *= d
    i += 1
  end while
end *=

// After
inline def *=(d: Double): Unit =
  blas.dscal(vec.length, d, vec, 1)
end *=