feat: export WrappingSet and narrow SetFrom's return type to WrappingSet<T>

[jeswr]

Makes WrappingSet part of the public API and lets the type system say so.

WrappingSet is the concrete type behind every set-mapped model property, but it was internal: consuming code could not name the type, and SetFrom.subjectPredicate could only advertise the plain Set<T> interface. This is a prerequisite for set-level features (e.g. the per-set change notifications explored in #73), which need the concrete class to be publicly nameable.

Changes

• src/mod.ts: export * from "./WrappingSet.js"
• src/mapping/SetFrom.ts: narrow subjectPredicate's return type from Set<T> to WrappingSet<T>. Non-breaking: WrappingSet<T> implements Set<T>, so every existing annotation (e.g. model properties typed Set<Child>) still type-checks; the runtime value is unchanged.
• src/WrappingSet.ts: class and constructor JSDoc (live-view semantics, SetFrom.subjectPredicate as the intended entry point, a paired turtle/ts example).
• test/unit/wrapping_set.test.ts: asserts a set-mapped model property is an instanceof WrappingSet, and (at compile time, via the model's WrappingSet<string> property type) that the narrowed return type is exposed through the package surface.

Relationship to #73

Supersedes one slice of the #73 omnibus: the SetFrom.subjectPredicate return-type narrowing and the mod.ts WrappingSet export (including the non-events portion of its WrappingSet JSDoc). The event-emission machinery itself (on/off, listener registry, notifying dataset) is intentionally not included here and remains for follow-up PRs.

Review timing: This draft was prepared with Claude; I (@jeswr) will personally review it before it progresses. I'm currently batching a lot of work in flight, so expect active review Wednesday-Friday (8-10 July).