Wake a pending shutdown when in-flight data is acknowledged

[FrenchToblerone54]

Problem

poll_shutdown sends the FIN only when no data is in flight. If there is some, it returns Pending, but the only waker it registers is the one woken when the session is fully shut down. The ACKs that settle the in-flight data wake write_notify, and nothing is registered there once the caller is shutting down instead of writing. The FIN is therefore never sent unless something else happens to poll the stream again, and the shutdown hangs.

Reproduce

Write a few bytes, call poll_shutdown (it returns Pending), then deliver the peer's ACK for those bytes. The shutdown task is not woken. The new test does this.

Fix

In poll_shutdown, when data is still in flight, register the task's waker on write_notify, which the ACK path already wakes. The next poll then sends the FIN.

Test

shutdown_is_woken_when_in_flight_data_is_acknowledged in src/stream/tcp.rs. It fails without the change ("the ACK did not wake the pending shutdown") and passes with it, and it checks that the FIN goes out on the following poll. cargo test and cargo clippy --all-targets are clean.

[FrenchToblerone54]

Reopened as #105 with a cleaned-up commit.