client: re-export blocking tick/display-group subscriptions

[tradatious]

Description

With both sync and async enabled, the top-level market_data::historical::TickSubscription and
display_groups::DisplayGroupSubscription are the async types. The blocking TickSubscription was
reachable there only through the hidden historical::sync path, and the blocking
DisplayGroupSubscription had no public path at all. So a caller of
blocking::Client::subscribe_to_group_events could not name the returned type, and a caller of
historical_ticks(..).trade() could name it only through a hidden module.

client::blocking now re-exports the blocking TickSubscription, its four iterator types and
DisplayGroupSubscription, as docs/rules/parity/dual-feature-types.md prescribes for dual-feature
types. The blocking module docs gain an example that names both. The hidden historical::sync
path is unchanged. The rule node records this as a precedent.

Testing

The new example on client::blocking compiles under --all-features and sync-only; without the
re-exports it fails with E0432. cargo fmt --check; clippy, rustdoc (-D warnings) and cargo test on
all three feature configurations; cargo build --examples both ways; integration crates built; rules
graph check.

Breaking changes

No. Additive re-exports only.