orders: drop builder::{OrderBuilder, BracketOrderBuilder, BracketOrderIds, OrderId} duplicate paths (#602)

* orders: drop builder::{OrderBuilder, BracketOrderBuilder, BracketOrderIds, OrderId} duplicate paths

These four types were reachable at both `orders::*` (canonical) and
`orders::builder::*` (duplicate via `pub use order_builder::{...}` /
`pub use types::{...}` inside builder/mod.rs). Zero external callers
of the duplicate path; only crate-internal tests + sync/async impl files
referenced it, updated to canonical `orders::*` / `super::order_builder::`
/ `super::types::`. The low-level fluent layer (orders::builder::price,
algo builders, etc.) is unchanged.

`order_builder` and `types` submodules narrowed from `mod` to `pub(crate) mod`
so orders/mod.rs can re-export through them. Internal rustdoc link in
orders/common/order_builder/mod.rs:31 updated.

Plan: plans/one-way-to-spell.md (PR 3 of 5). Migration §22 added.

* orders: trim narrative comments (/simplify)

- Drop orphan back-pointer at builder/mod.rs:28 (stapled onto unrelated
  pub use types::{...} line; referenced names not in adjacent statement).
- Tighten orders/mod.rs:45 comment: state invariant instead of narrating
  the one-way-to-spell deletion; drop transient rule reference.