Fix Matrix.fromColumns column/row indexing and add regression tests

[Ezrnest]

Motivation

• Matrix.Companion.fromColumns was constructing matrices with the wrong index order, causing (row, col) to read from the wrong element of the supplied column vectors.

Description

• Corrected Matrix.Companion.fromColumns to build entries with columns[j][i] so that matrix[i, j] equals the i-th element of the j-th input column vector (src/main/kotlin/.../Matrix.kt).
• Added a regression test fromColumnsKeepsColumnMajorPlacement that constructs 3 column vectors with distinguishable values (10 * col + row) and asserts matrix[row, col] == columns[col][row] (src/test/kotlin/linear/MatrixTest.kt).
• Added a rectangular (2x3) regression test fromColumnsKeepsColumnMajorPlacementForRectangularMatrix which asserts dimensions and per-element mapping to prevent square-matrix masking of indexing errors (src/test/kotlin/linear/MatrixTest.kt).

Testing

• Attempted to run sh gradlew test --tests 'linear.MatrixTest', but the Gradle wrapper failed to download due to a network connection error to services.gradle.org, so tests could not be executed in this environment.
• The change is minimal and covered by the added unit tests which should pass once the test environment can run Gradle.

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Codex Task