cg-shuffle gains an optional SEED and a `cg-shuffle-seed' dynamic var.
With a seed the order is deterministic and identical on every platform,
so a deal can be replayed, shared between players, or pinned in a test;
the index comes from secure-hash (spec-defined output everywhere, 7 hex
digits stay a fixnum even on 32-bit Emacs), so no bignum/overflow
behaviour can vary it. With no seed the shuffle is the byte-for-byte
old system-random path -- normal play is unchanged.
Test cgt-shuffle-seeded pins determinism, permutation, override and a
cross-platform golden prefix; fails on the prior one-arg cg-shuffle.
Suite 159/159.
