SY-3833-1: Add Pluto Flux Selector Primitive

Adds Flux.createSelector, a generic React hook factory for subscribing
to scoped flux store state with referential stability. Supports custom
equality, args memoization via useMemoDeepEqual, and version-bumped
caching to prevent infinite re-renders when select returns derived
objects.

Author: emilbon99

pluto/src/flux/external.ts

@@ -15,4 +15,5 @@ export * from "@/flux/list";
 export * from "@/flux/Provider";
 export * from "@/flux/result";
 export * from "@/flux/retrieve";
+export * from "@/flux/select";
 export * from "@/flux/update";