The developer of Boston’s most ambitious real estate undertaking ever—the planned $3-billion Seaport Square—expects to select a construction manager for the 23-acre minicity early next year, assuming it receives approval later this year from the city’s zoning commission. Construction is expected to begin in earnest next fall on the first two 500,000-sq-ft residential towers on the 20-block site. The project, which would be phased over a decade, is expected to create 10,000 construction jobs. Sited in the South Waterfront Innovation District, Seaport Square received approval last month from the Boston Redevelopment Authority. The master plan by Kohn Pedersen Fox Associates calls for 6.3 million sq ft of mixed-use development in 22 buildings. Several architects are involved, including Studio Daniel Libeskind, HOK, CBT Architects and Hacin + Associates.