Axis parameter is a u8 magic number in from_iterator and related APIs

[genefold-ai]

Problem

DenseMatrix::from_iterator(iter, nrows, ncols, axis: u8) (and related axis-taking APIs) use u8 magic numbers: 0 = row-major fill, 1 = column-major fill.

Two failure modes observed downstream (genefold test suite):

1. Accidental transposition — passing 1 where 0 was meant silently transposes the matrix; dimensions still match, values are wrong. This actually happened across fixtures in the same codebase.
2. Undocumented convention — the meaning of 0/1 is only discoverable from source; nothing in the type prevents 2.

Proposal

pub enum Axis { Row = 0, Col = 1 }

impl DenseMatrix<T> {
    pub fn from_iterator_axis(iter: I, nrows: usize, ncols: usize, axis: Axis) -> Result<Self, Failed>;
}

• New enum-taking methods first (no breakage of existing u8 signatures, deprecate later);
• TryFrom<u8> for Axis so stringly/CLI boundaries convert with an error instead of a wrong-number bug.

A repo-wide grep shows the axis: u8 convention repeats across other constructors/iterators — worth treating uniformly.