fix(combinatorics): stirlingS2 returns plain number instead of BigNumber for S(n,n) with n>0

[JSap0914]

Bug

stirlingS2(bignumber(n), bignumber(n)) returns a plain JS number (1) instead of a BigNumber when n > 0, even though both inputs are BigNumbers. The S(0,0) case is correct because it is initialised via cache[0] = [make(1)] (with make = bignumber in BigNumber mode). However, the diagonal entries for n > 0 are filled by:

if (i === m) {
  row[i] = 1          // BUG: plain literal, not make(1)
}

so the result is always the primitive 1 rather than bignumber(1).

Reproduction:

math.typeOf(math.stirlingS2(math.bignumber(3), math.bignumber(3)))
// 'number'  ← should be 'BigNumber'

Fix

Replace row[i] = 1 with row[i] = make(1) so the diagonal entry is created with the same factory function used everywhere else in the loop.

Verification

npx mocha test/unit-tests/function/combinatorics/stirlingS2.test.js
# 5 passing

Four regression tests added: stirlingS2(bn(1),bn(1)), stirlingS2(bn(3),bn(3)), stirlingS2(bn(7),bn(7)), and the mixed-input stirlingS2(bn(4),4) — all now return BigNumber(1).

AI-assisted contribution

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