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feat: add `order_terms` for user-controlled term ordering (#184)

`canonical_order` imposes a single fixed order, but the order of an
operator's terms is otherwise dictated by how it happened to be
assembled. Since term iteration order can materially affect runtime
algorithm behavior (e.g. randomized product-formula sampling in the
qDRIFT pass, cf. #164), users need a way to impose an arbitrary,
reproducible order of their own.

`order_terms(op, key, *, reverse=False)` does this: it sorts the terms
by a `sorted`-style key function and rebuilds a new operator of the same
type, leaving the input untouched. It is pure Python (like the `mappers`
framework helpers) and works for any built-in operator type via the
`OperatorTrait` protocol, so it needs no per-type dispatch and has no C
API counterpart. Group indices are preserved: when the operator is
grouped, the key receives the `(actions, coeff, group)` tuple and each
term carries its group tag as it moves.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

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