Fixes #3487

Problem

On Windows, typing exit or quit in a UCM shell session would cause the terminal to hang, requiring a manual Ctrl+C to kill the process. This was caused by the GHC Windows I/O manager being unable to deliver async exceptions to threads blocked in a socket accept call. When the UCM CLI exited, Ki.scoped would try to cancel and join the LSP thread, but the LSP thread was blocked in TCP.serve waiting for a connection and would never wake up.

As a workaround, the LSP was disabled by default on Windows behind a UNISON_LSP_ENABLED=true environment variable flag.

Fix

Instead of using TCP.serve (which owns the socket internally), we now manually bind the socket with TCP.bindSock so we can hold a reference to it. The socket is passed back to Main.hs via a callback. After the UCM CLI exits, Main.hs closes the socket, which unblocks the accept call at the OS level and allows the LSP thread to exit cleanly on all platforms.

A shutdownVar flag is set before the accept loop starts. When handleFailure catches an IOException during shutdown, it checks this flag and suppresses the misleading "LSP server failed to start" message. All other error cases should be unaffected, since only IOException is caught here, and only the error message is suppressed, not any other behavior.

With the exit hang fixed, the UNISON_LSP_ENABLED Windows guard in ifEnabled has been removed, enabling LSP on Windows by default.

Result

UCM now exits cleanly on Windows without hanging, and the LSP server starts by default on Windows, allowing the VS Code extension to connect without any additional configuration.