A single track of a `Flow.grid` template. `Fixed` tracks take literal cells; `Auto` tracks size to the largest footprint of their span-1 places at construction (a context-sized place reports no footprint, so it contributes nothing), and a place that spans several tracks shares its footprint over the `Auto` tracks it covers; `Weight` tracks (`fr`) share the space left after `Fixed`, `Auto` and `Percent` tracks; `Percent` is a fraction of the assigned container length.