[bsergean]

A failed send currently invokes Close on the sending thread while the connection thread may still be inside a Message callback. This can overlap callbacks and deliver messages after Close.

Use the existing close path: fatal writes, write-readiness errors, and send timeouts call close(1006, reason). An abnormal close discards pending output and wakes the connection thread without sending a Close frame. The active Message callback finishes; further buffered data callbacks are suppressed while CLOSING, as with an explicit close. The connection thread closes the socket and delivers the final Close callback. CLOSED is published before the callback so reentrant close() is safe.

This uses the existing state, close code, and close reason, with no additional member variables. The first close request retains its code/reason, and timeout/cancellation behavior remains covered by tests. Production code remains C++11 compatible.

Closes #551.

Validation:

• Full OpenSSL suite: 25/25 passing. Full Mbed TLS suite: 26/26 passing.
• Regression covers client/server failures, write-readiness errors, timeouts, callback ordering/thread, reentrant close(), cancellation, close before connecting, and reconnect. It passes under ThreadSanitizer.
• Reporter’s server under ThreadSanitizer with 20 abrupt disconnects during bidirectional traffic: passing.
• Targeted Autobahn: 51/51 cases passing in each role. Full compliance requested through the autobahn label.