After raising $400 million in September in a round that valued it at $4.4 billion, real estate startup Compass is making an acquisition to help it double down on growth: it is acquiring a portion of Spring, the mobile-first fashion shopping app that has raised more than $100 million since 2014, to work on parts of Compass’s AI-based real estate platform, which provides a marketplace to post properties for sale and rental across the US and soon other countries.