gui_toolkit 0.43.0

Added

• platform.Frame_Gate: the frames of a thread as one state machine (platform/frame_gate.odin, docs/frame_gate.md): Idle -> Polling -> Work -> Building -> Drawing -> Presenting, Full and Light frames, frame_gate_begin / frame_gate_stage / frame_gate_end; a frame asked for where none may begin is deferred, never nested, and a move the machine does not have is counted (a panic with strict). The window procedure defers a WM_PAINT where no frame may begin; drag_script pumps the thread's messages between its steps as the system's drag loop does. Test: tests/frame_gate.