Add a SetVar backed by the JS Set object #7623

Summary

Python set values are not well supported as vars today. ArrayVar registers set among its Python types, so a set-typed state var or a set literal is treated as a plain sequence: it renders as a JS array and only array operations (length, indexing, .map, contains) are available. frozenset is not accepted at all (LiteralVar.create(frozenset(...)) raises TypeError). #7622 briefly added frozenset to ArrayVar and was reverted in #7632, because widening the array types is the wrong direction.

Proposal

Add a dedicated SetVar (and LiteralSetVar) for set[T] and frozenset[T] that compiles to a real JS Set and exposes set operations rather than array operations:

• membership: contains / has
• size: length / size
• combination: union, intersection, difference, symmetric_difference, issubset, issuperset
• conversion: to_array (for rx.foreach and anything that wants a sequence)
• the operations should return SetVar or BoolVar as appropriate, with element type T preserved statically, so LiteralVar.create({"a"}) is typed as SetVar[set[str]] rather than a plain Var.

Once SetVar exists, remove set from ArrayVar's Python types and make figure_out_type / _determine_value_of_array_index route set and frozenset to the new class.

Prerequisite: rehydrating Set on the frontend

State values travel to the frontend as JSON, and serialize_set turns a set into a list. There is no mechanism today to turn a JSON-encoded value back into a real Set object on the frontend, so a set-typed state var arrives as an array and a SetVar operating on it would have nothing to call .has() on.

Before SetVar can work on state vars, the frontend needs a way to rehydrate JSON values as Set objects, driven by the var's declared type (for example a per-var deserializer applied in the state delta handler, or a tagged encoding chosen on the backend). Design that first; SetVar on literals alone is not very useful.

Not in scope

Whether a set var should satisfy a Sequence[...]-typed component prop is a separate question (typehint_issubclass / _isinstance in reflex_base/utils/types.py). With SetVar the answer is probably "no, call .to_array()", but decide it separately.

Longer-horizon item; no implementation planned yet.