OrderedBasis

The "leading term" (highest-priority cell and its coefficient, under CellT's own order) a formal sum needs to support pivot-based reduction. Currently has exactly one instance in this codebase, Chain's own chainIsOrderedBasis (Chain.scala) -- kept as a separate typeclass, in the same is-typeclass style as Cell/OrderedCell, rather than folded into Chain as ordinary methods, so leadingCell/leadingCoefficient read as a documented contract rather than incidental Chain API.