perf: stack-backed storage for exact arithmetic kernels

[acgetchell]

Summary

Replace Vec<Vec<BigRational>> with [[BigRational; D]; D] in bareiss_det and gauss_solve to eliminate heap allocation for small dimensions (D ≤ 5).

Current State

• bareiss_det (src/exact.rs:91) allocates a Vec<Vec<BigRational>> — one heap allocation per row, one per entry, plus pointer chasing.
• gauss_solve (src/exact.rs:157) allocates a Vec<Vec<BigRational>> for the augmented D×(D+1) matrix.
• For small D (which dominates geometric predicates), allocation cost is very expensive relative to the math.

Proposed Changes

bareiss_det

Replace:

let mut a: Vec<Vec<BigRational>> = Vec::with_capacity(D);
for r in 0..D {
    let mut row = Vec::with_capacity(D);
    for c in 0..D { row.push(f64_to_bigrational(m.rows[r][c])); }
    a.push(row);
}

With:

let mut a: [[BigRational; D]; D] = std::array::from_fn(|r| {
    std::array::from_fn(|c| f64_to_bigrational(m.rows[r][c]))
});

Row swaps use .swap() on the outer array (same as Vec::swap).

gauss_solve

Cannot use [BigRational; D+1] (const-generic expressions are unstable). Instead, split into:

• [[BigRational; D]; D] for the matrix part
• [BigRational; D] for the RHS vector

Operate on both during elimination and back-substitution.

solve_exact return

The gauss_solve return type changes from Vec<BigRational> to [BigRational; D], eliminating the clone in solve_exact line 305:

Ok(std::array::from_fn(|i| solution[i].clone()))

Benefits

• Eliminates all heap allocation for D ≤ ~20
• Dramatically improves cache locality
• Stack frame grows as D² × sizeof(BigRational) — fine for D ≤ 5

Constraints

• Crate uses #![forbid(unsafe_code)] — all changes must be safe Rust only
• std::array::from_fn is the correct safe approach (no MaybeUninit)

Implementation Notes

• All existing tests must continue to pass
• Add benchmarks for exact arithmetic to measure improvement
• Depends on: none
• Blocks: Bump actions/upload-artifact from 5.0.0 to 6.0.0 #2 (custom f64 conversion), Changed: Overhauls public API tests and examples #3 (integer Bareiss)

[acgetchell]

Blocks #63, #64