fn engine_model(model: &Model) -> Cow<'_, Model>Expand description
model as the ENGINE must see it: every measure whose formula is disabled
flattened to MeasureKind::Input, keeping its (still-stored) formula out of
the dataflow entirely.
This is the single point that makes disabling real rather than cosmetic. A
flattened measure leaves derived_build_order, so it computes nothing (its
cells go blank) and every dependent recomputes against its absence instead
of through it.
Borrows when nothing is disabled — the overwhelmingly common case — so the projection costs nothing on models that never touch the feature.