Split Builder along the seam its two users already follow

[frankmcsherry]

Builder had two consumers using disjoint halves of it.
MergeBatcher called only seal, and through it with_capacity, because it holds a whole chain and can count keys, values, and updates before allocating.
Reduce and upsert called only new, push, and done, discovering their output key by key and never sizing anything.

seal moves to a trait of its own:

pub trait Sealer<C> {
    type Output;
    fn seal(chain: &mut Vec<C>) -> Option<Self::Output>;
}

which is a fn(&mut Vec<C>) -> Option<B> in a form that can name B.
It has no receiver, so there is nowhere for an update to be retained between the chain going in and the batch coming out.

with_capacity becomes inherent on the two ord_neu builders.
It had no callers outside seal and new, so it was never vocabulary; new now carries its own one-line body instead of a default.

MergeBatcher bounds Bu: Sealer<M::Chunk> and stops naming Builder, which also drops Builder::Time from its vocabulary — it never wanted it.
The three implementors implement both traits and share their machinery.

No call site changes: the aliases name the same types, and those types satisfy both bounds.
The diff is four files, all under trace/.

This splits one question into two.
Whether reduce and upsert should take a Batcher instead, and whether Sealer should be a function value rather than a trait, can now be answered independently.

Builds warning-free, workspace tests and doctests pass.