Explain how `next(err)` works to forward errors to the global error handler in Express.js.

The `next(err)` function call is the primary mechanism in Express.js for explicitly forwarding an error to the global error handling middleware. When `next()` is called with an argument (conventionally an `Error` object), Express identifies it as an error condition. It then bypasses all subsequent non-error-handling middleware and route handlers in the current request-response cycle and proceeds directly to find the next error-handling middleware (one with four arguments: `err, req, res, next`). This allows for centralized error management, where specific errors can be caught, wrapped, or augmented at various stages of the request and then uniformly handled by a global handler at the end of the chain.