Metric Path

For the metric-series bands: a `tx.Expression` evaluated against each tracked sub-resource (bound to the `input` variable) to the numeric value plotted. A direct field is a `tx.PropertyRead` over a `tx.VarRef` of `input`; a reference step composes `tx.Traverse` (through the reference property) with a `tx.PropertyRead` for the final field. Reading the mean of a thesis's distributional confidence estimate, for example: `tx.Traverse{ tx.traverseSource: tx.VarRef(input), tx.through: wv.confidence, tx.step: tx.PropertyRead{ tx.readSource: tx.VarRef(input), tx.readProp: est.estimateMean } }`. Every property reference resolves through the object model and validates.