Function gen_selection

pub fn gen_selection(
    depth: u32,
    shape: Vec<usize>,
    dim: usize,
) -> BoxedStrategy<Selection>

Recursively generates a random Selection expression of bounded depth, aligned with the given slice shape.

Each recursive call corresponds to one dimension of the shape, starting from dim, and constructs a selection operator (range, all, intersection, etc.) that applies at that level.

The recursion proceeds until either:

• depth == 0, which limits structural complexity, or
• dim >= shape.len(), which prevents exceeding the dimensionality.

In both cases, the recursion terminates with a true_() leaf node, effectively selecting all remaining elements.

The resulting selections are guaranteed to be valid under a strict validation regime: they contain no empty ranges, no out-of-bounds accesses, and no dynamic constructs like any or first.