Interface Tangible

Read the Stuff's mass as a Quantity<'kg'>. Strict on the runtime type — the marshaller absorbs persistence-shape coercion at the hydration boundary.

Resolve the Material at detailKey, falling through to the bulk default when no per-Detail override is set. Omit detailKey to read the bulk default directly.

Set the Stuff's mass. Strict on Quantity<'kg'>; callers holding a raw number wrap via Quantity.of(n, 'kg'). YAML authoring shapes (tag string, alt-unit literal, bare number) are absorbed by the QuantityMarshaller for kg, not by this runtime API.

Set the Material at detailKey, or the bulk default when detailKey is omitted. setMaterial(null, detailKey) removes the per-Detail override (later reads at that key fall through to the bulk default again). setMaterial(null) clears the bulk default AND every per-Detail override.