Allow plain Int for row/column in Matrix slice syntax

[Copilot]

mat(::, Array(0)) worked but the more natural mat(::, 0) did not, requiring users to unnecessarily wrap single-index selections in Array(...).

Changes

• MatrixInstance.scala: Two new apply overloads that accept a plain Int for either dimension alongside a RangeExtender, delegating to the existing Array-based slice implementation:
  • apply(rowRange: RangeExtender, col: Int)
  • apply(row: Int, colRange: RangeExtender)

No changes to RangeExtender itself — avoids ambiguity with the existing element-access apply(row: Row, col: Col): A.

Example

val mat = Matrix.fromRows(
  Array[Double](1.0, 2.0, 3.0),
  Array[Double](4.0, 5.0, 6.0),
  Array[Double](7.0, 8.0, 9.0)
)

mat(::, 1)    // column 1 — equivalent to mat(::, Array(1))
mat(1, ::)    // row 1    — equivalent to mat(Array(1), ::)
mat(0 to 1, 1) // rows 0–1, column 1
mat(1, 0 to 1) // row 1, columns 0–1