Cardinality

The second main dimension. Where mutation asks does this touch Context, cardinality asks how many values does it yield.

Definition

Each atom yields in one of three shapes:

• SCALAR - one value.
• STREAM - 0..N values.
• VOID - nothing.

Plus one polymorphic case:

• TRANSPARENT - forwards whatever the child yields (Span).

What this forces

Yielding kinds split by cardinality:

• Query -> ScalarQuery (one value) or StreamQuery (many).
• Action -> ScalarAction (one value) or StreamAction (many).

Refs are SCALAR (one address, one value). Commands and Flows are VOID (no yield, so the split doesn't apply). Span is TRANSPARENT (forwards its body).

Consequence

A scalar consumer can't honestly accept a stream producer - there's no canonical reduction (first? last? sum? collect?). The choice belongs to the consumer, named explicitly (First, Collect, Reduce(op, init)). Full composition matrix and reduction laws come later.