Keep one frame on the project mood board.

Pin one frame as part of the agreed look, so every character render afterwards is made in it. Costs no credits and generates nothing: the picture already exists and this writes it into canon. Keeping a frame whose id is already kept replaces that entry. Keeping a fifth distinct frame is REFUSED with 409 mood_board_full and nothing is trimmed — four is the most that can bind a render, so a fifth frame would silently have no effect. A project that has not passed premise coverage is refused with 409 project_precondition_failed.