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Add combinatorics functions: factorial, perm, catalan, multinomial

Adds a new combinatorics module: factorial(n) (a named equivalent of the
existing postfix ! operator, usable as an ordinary function argument),
perm(n, r) for permutation counts, catalan(n) for Catalan numbers, and
multinomial(n, k1, k2, ...) generalizing binom to more than two groups.

All guard against overflow via incremental multiplication with an
infinity check, mirroring the existing factorial/binom implementations.
New docs/functions/combinatorics.md, README examples, and CHANGELOG entry;
function count 160 -> 164.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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