fix: add recurrent graceful close for raw TCP

[kentbull]

What

• Add recurrent graceful close for raw Client and Remoter.
• Drain queued output before issuing local SHUT_WR.
• Wait until local receive observes peer write EOF before final disposal.
• Retain write-shutdown failures and keep TLS on its separate force-close path.

Why

Immediate close can discard accepted output. Raw TCP shutdown is directional: graceful close must settle egress before local write shutdown and keep local receive available for final peer input.

Background

This is a recurrent transport primitive, not an owner state machine. Higher-level owners remain responsible for protocol completion, continued service, deadlines, and choosing force-close when settlement cannot finish.

Boundary

This PR does not define HTTP framing policy, owner deadlines, or TLS close_notify shutdown.

[kentbull]

Upstream prerequisite: an accepted directional-state and terminal-cause contract for Client and Remoter, equivalent to #9/#10 and the Remoter behavior proposed by ioflo#163.

This raw TCP behavior does not depend on #11/#12's TLS classifications. It can be adapted once queued output drains independently of local receive EOF and local write shutdown can be represented without conflating both directions.

[kentbull]

Implements downstream behavior for ioflo#175 by adding recurrent graceful close for raw Client and Remoter: drain accepted output, issue local SHUT_WR, wait until local receive observes peer write EOF, then dispose of the socket. #16 adds real-socket coverage.