Async SHM multi-server path is non-functional

[mcurrier2]

Description

In shared_memory.rs, the multi-server implementation:

• Registers connections but never reads or forwards messages from them
• handle_connection inserts into the connections map but has no receive loop
• start_multi_server monitoring task never delivers messages to the channel

Additionally, close() does not set the ring buffer shutdown flag, causing blocked peers to spin until the 5-second timeout.

Impact

Multi-client shared memory mode (if invoked via the trait interface) would hang indefinitely with no messages delivered.

Suggested Fix

1. Add a receive loop in handle_connection that calls receive_message() and sends on message_sender
2. Set shutdown flag in close() (mirror shared_memory_blocking.rs:848–857)

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Found during release polish review