protocol: resolve TLS transport from the bound session, not thread-local io_ssl

file_send.c chose between sendfile() and the TLS-aware buffered path by
calling io_get_ssl(), which reads the thread-local io_ssl. A worker thread
that bound a TLS ProtocolSession via protocol_session_bind() never ran the
handshake in that thread, so io_ssl is NULL there and a TLS + --threads
transfer took the raw sendfile() path on an encrypted socket.

Add protocol_current_ssl(), which prefers the bound session's SSL and falls
back to io_ssl on the fd-shim path, and use it in file_send.c. Un-xfail
test_tls_with_multithreading.

tests/integration/test_tls.py

@@ -146,7 +146,6 @@ class TestTLSBasic:
         assert not missing, f"Missing files: {missing}"
         assert not mismatches, f"Mismatched files: {mismatches}"
 
-    @pytest.mark.xfail(reason="TLS multithreading has architectural limitations with per-thread SSL context")
     def test_tls_with_multithreading(self, certs):
         """TLS + multithreading."""
         clean_dir(DEST_DIR)