fix: preserve Client sends after peer write EOF

[kentbull]

What

• Track Client receive and transmit terminal state independently.
• Retain terminal send causes and unsent bytes.
• Reject output after transmit closure.
• Reset terminal facts only for a newly allocated socket.

Why

Local receive EOF caused by peer write shutdown is directional and must not suppress Client transmission. Conversely, a terminal send failure must stop impossible retries without deleting accepted output or final peer input.

Boundary

This PR changes raw Client state and enqueue behavior only. It does not change Remoter, TLS classification, connection-wide Client send classification, or graceful shutdown.

[kentbull]

Upstream dependency: this is the first Client-side directional-state change in #9–#17. It introduces Client.txCutoff, retained terminal causes, and TransmitClosedError; ioflo#163 currently proposes only the corresponding Remoter-side foundation.

This behavior must be adapted to the directional-state abstraction accepted on current upstream main; it is not a clean cherry-pick.

[kentbull]

Implements downstream behavior for ioflo#171.

The fork Client can preserve and send output after local receive EOF caused by peer write shutdown. #16 adds the corresponding real-socket regression coverage.

[kentbull]

Partially implements the Client portion of ioflo#172 by retaining terminal send causes and unsent bytes and rejecting later output after transmit closure.

Connection-wide Client send errors in this PR still close only transmit, so the complete upstream contract remains open. #10 implements the corresponding Remoter behavior.