cow-venue: reconsider the OrderBuilder typestate

[mfw78]

From review of #461 (r3616036865). Confirmed still present at the end-of-train tip. Target: cow-venue (L3). Not a defect; a question about whether the machinery earns its keep.

OrderBuilder<S> in cow-venue/src/order.rs is a typestate builder: a PhantomData<S> state parameter, four state markers (NeedsBuy, NeedsSell, NeedsValidTo, Ready), per-state impl blocks, and build available only on OrderBuilder<Ready>.

The question

OrderBody's required fields (sell and buy side plus amount, valid_to) are flat and mutually independent. Nothing in CoW's wire format constrains the order in which they are set, only that all of them end up set. Typestate earns its cost when call order is semantically constrained, for example must-sign-before-submit. Here the invariant is really "do not forget a required field", which is a weaker claim than the machinery expresses.

Options

• Keep it. The compile-time completeness guarantee is real, and for a financial order type a missing required field is worth catching at compile time rather than runtime. If kept, say so in the type's rustdoc so the next reader does not re-open this.
• Required args in the constructor, optional setters after. OrderBuilder::new(sell, buy, valid_to) followed by optional setters keeps the same compile-time completeness guarantee with no phantom states, no per-state impl blocks, and no duplicated method surface. This is the strongest alternative: it does not trade safety for simplicity, it gets both.
• Result-returning build. Simplest surface, but moves a caught-at-compile-time error to runtime. Weakest option for this domain; listed for completeness only.

Acceptance criteria

Either the typestate is replaced by required-args-in-constructor with the completeness guarantee preserved, or the typestate stays and its rustdoc states why the state machine is worth the surface.

[mfw78]

Decision (mfw78, 2026-07-22): replace the typestate with a required-args constructor.

OrderBuilder::new(sell, buy, valid_to) plus optional setters. Keeps the same compile-time completeness guarantee while dropping PhantomData, the four state markers, the per-state impls and the duplicated method surface. Not the keep-and-document option.

Because this rewrites the same signatures in cow-venue/src/order.rs that #546 rewrites, settle this before #546 so the file is reworked once.