PERT / CPM network

Native project network diagrams with the critical path highlighted. Declare tasks with `duration` and `after`; the engine runs the full forward + backward pass and renders ES / EF / LS / LF / slack in every node. Zero-slack tasks are flagged as CRITICAL in amber. Finish-to-start only, in whole unit-less days — for a schedule on a real working calendar, with start-to-start, finish-to-finish and start-to-finish relationships, lead and lag, resources, deadlines and the compression ranking, use `gantt`, which draws the same network under `view network` from a schedule it also draws as bars.