client: drop duplicate Subscription re-export; consolidate prelude (#571)

* client: drop duplicate Subscription re-export; consolidate prelude on subscriptions

`ibapi::client::Subscription` was a duplicate of `ibapi::subscriptions::Subscription`
with zero in-crate consumers — only `prelude.rs:51` reached in via that path.
Drop the `client/mod.rs` re-exports (5 lines) and rewire prelude to source
from `crate::subscriptions` directly. The async path already did; the sync
path's `client::Subscription` indirection was the asymmetry called out in
v3-api-ergonomics.md §2.

Net public-API delta (v3.0):
- Removed: `ibapi::client::Subscription` (duplicate alias).
- Unchanged: `ibapi::subscriptions::Subscription` (canonical), `ibapi::prelude::Subscription`
  (prelude convenience, now symmetric), `ibapi::client::blocking::Subscription`
  (labelled sync-explicit path; needed when both features are enabled).

The `subscriptions::Subscription` re-export is itself feature-gated at the
source (`subscriptions/mod.rs:32,35` — sync when not-async, async otherwise),
so the prelude re-export collapses to a single unconditional line.

Migration note added at docs/migration-3.0.md §13.

Detailed plan: plans/subscription-canonical-import-path.md.

* /simplify pass: collapse prelude orders re-exports; trim plan drafts

Two findings applied from a code-reuse + quality review pass:

1. `pub use crate::orders::{...}` had two identical RHS lines gated by
   mutually exclusive `cfg`s (sync-only vs async) — same anti-pattern
   this PR just fixed for `Subscription`. Collapsed to a single
   unconditional `pub use` since `lib.rs`'s `compile_error!` guarantees
   at least one feature is on.

2. The plan file duplicated the migration-3.0 §13 draft and a concrete
   change sketch. Now that the migration note shipped, the draft has
   drift risk — replaced with a one-line redirect. Concrete-change
   sketch left in place as historical context for the PR.

Also tracked in v3-api-ergonomics.md §2: a follow-up to consolidate
`SharesChannel`'s triple re-export — same shape, separate scope.

Skipped finding: prelude cfg ordering (rustfmt normalizes the order
itself, overriding the human-readable convention).