fix: retain Remoter terminal causes and queued output

[kentbull]

What

• Retain the transport exception that closes a Remoter direction.
• Preserve unsent bytes.
• Reject later enqueue attempts with a chained TransmitClosedError.

Why

Local receive EOF caused by peer write shutdown does not make transmission unusable. A terminal send failure does. Keeping the directions and original cause separate lets an owner drain final input without silently accepting output that can no longer be sent.

Boundary

This PR gives accepted raw TCP connections terminal-cause and late-enqueue behavior corresponding to Client. It does not classify TLS outcomes or implement graceful close.

[kentbull]

Upstream prerequisite: the Remoter directional half-close foundation in ioflo#163, or an accepted replacement, plus the shared terminal-cause and late-enqueue contract introduced by #9.

Without those already on upstream main, this PR’s Remoter.txCutoff, retained error, and chained TransmitClosedError behavior has no upstream substrate.

[kentbull]

Implements the downstream Remoter portion of ioflo#172 by extending the terminal-cause, unsent-output, and late-enqueue contract from #9. The upstream issue remains broader, including complete Client connection-wide failure classification.