Possible bug in outerProduct

[dsyme]

From #17

Potential Bug: outerProduct Implementation Incomplete
Location: src/FsMath/SpanMath.fs:338-353

Issue: The outerProduct function only implements the SIMD path and doesn't have fallback logic for remaining elements:

for i = 0 to rows - 1 do
    let ui = u[i]
    for j = 0 to cols - 1 do
        // Only SIMD path implemented - no fallback for j loop!
        for k = 0 to simdCount - 1 do
            let vi = Numerics.Vector<'T>(ui)
            let res = vi * vVec[k]
            res.CopyTo(MemoryMarshal.CreateSpan(&data.[i * cols + k * simdCols], simdCols))

Impact: For vectors smaller than SIMD width, or for remaining elements after SIMD processing, the function returns zeros instead of computing the actual outer product.

Example: outerProduct([1.0; 2.0], [3.0; 4.0; 5.0]) returns all zeros instead of the expected [3, 4, 5, 6, 8, 10].

Recommendation: Add a fallback loop to handle elements not processed by SIMD:

// After SIMD loop
for j = ceiling to cols - 1 do
    data.[i * cols + j] <- ui * v.[j]
Tests for outerProduct were intentionally omitted from this PR to avoid failing tests on existing bugs.

[dsyme]

An AI-proposed fix to this is here: #18

It is bundled with a SIMD perf optimization so maybe worth separating out