Function place 

pub fn place(
    alloc: &Allocation,
    t: &PlasticineTarget,
    budget: LinkBudget,
) -> Result<Placement, Unplaceable>

Place the allocation on the checkerboard in pipeline order (a row-major snake over PCU cells, each stage’s chain contiguous, each edge’s PMU(s) taken from the PMU cells nearest the writer’s tail), then route every edge X-Y and count link occupancy against the budget.