Map branded primitives to their carrier type

[paoloanzn]

A branded type is a primitive joined to a marker object, for example string & { readonly __brand: "UserId" }. The marker exists only in the type system; at runtime the value is an ordinary string. The compiler had no rule for this shape, so it failed in two different ways: brands written as an object literal were treated as records and reported SC2006, and brands written with a unique symbol key reached the unresolved-intersection fence and reported SC2008.

Map these types to the primitive they wrap. When an intersection has exactly one primitive part, and every other part is a plain object type that adds nothing at runtime, the whole type now maps to that primitive. A branded value is stored, passed and computed exactly like an unbranded one. Reading a brand member is still refused at the point of use, like any other type-only member.

Two shapes stay refused because they describe no runtime value: a primitive joined to a class instance, since nothing can be both, and an intersection of two different primitives. The test for a type-only object part already existed inline in the handle-intersection path and is now a shared helper, so both paths use one definition. The primitive lookup is shared with the scalar mapping for the same reason.

A new differential program covers string and number brands, unique symbol tags, string methods and arithmetic on branded values, branded fields in interfaces, arrays, Maps and Sets, and generic passthrough. The intersection diagnostic fixture now uses a primitive joined to a class, which is the case that still cannot compile.

Closes #282

Written with AI assistance and reviewed by a human.

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