runtime: implement ES Set composition methods

[andrewtdiz]

Summary

Node exposes the ES Set composition methods on Set.prototype, but Perry does not implement dispatch/runtime behavior for them:

• union(other)
• intersection(other)
• difference(other)
• symmetricDifference(other)
• isSubsetOf(other)
• isSupersetOf(other)
• isDisjointFrom(other)

These are now part of the modern JS runtime surface and are used by newer libraries that target current Node.

Expected Node behavior

Local Node v25.9.0 probe:

for (const m of [
  "union",
  "intersection",
  "difference",
  "symmetricDifference",
  "isSubsetOf",
  "isSupersetOf",
  "isDisjointFrom",
]) {
  console.log(m, typeof Set.prototype[m]);
}

const a = new Set([1, 2]);
const b = new Set([2, 3]);
console.log([...a.union(b)]);                // [1, 2, 3]
console.log([...a.intersection(b)]);         // [2]
console.log([...a.difference(b)]);           // [1]
console.log([...a.symmetricDifference(b)]);  // [1, 3]
console.log(a.isSubsetOf(new Set([1, 2, 3]))); // true
console.log(a.isSupersetOf(new Set([1])));     // true
console.log(a.isDisjointFrom(new Set([4])));   // true

Current Perry behavior in source

docs/typescript-parity-gaps.md still lists these Set methods as missing.

crates/perry-codegen/src/expr/literals_vars.rs::is_set_method_name() includes the ES2024 composition names so property-value reads can look function-like:

"union"
| "intersection"
| "difference"
| "symmetricDifference"
| "isSubsetOf"
| "isSupersetOf"
| "isDisjointFrom"

However, source search found no corresponding HIR variants, no typed lowering in crates/perry-codegen/src/lower_call/property_get.rs, no crates/perry-runtime/src/set.rs helpers, and no dynamic branches in crates/perry-runtime/src/object/native_call_method.rs for these methods. That means the names may be visible as prototype method values, but actual calls do not have Node-compatible behavior.

Suggested tests

Add parity coverage for the basic Set inputs:

const a = new Set([1, 2]);
const b = new Set([2, 3]);
expect([...a.union(b)]).toEqual([1, 2, 3]);
expect([...a.intersection(b)]).toEqual([2]);
expect([...a.difference(b)]).toEqual([1]);
expect([...a.symmetricDifference(b)]).toEqual([1, 3]);
expect(a.isSubsetOf(new Set([1, 2, 3]))).toBe(true);
expect(a.isSupersetOf(new Set([1]))).toBe(true);
expect(a.isDisjointFrom(new Set([4]))).toBe(true);

Follow-up tests should cover set-like objects and TypeError cases once the basic Set-to-Set behavior is wired.

Scope / non-goals

This is scoped to exposing and dispatching the ES Set composition methods with Node-compatible basic behavior. Iterator object shape for existing keys / values / entries remains separate (#2856), and Set insertion-order preservation after delete is tracked in #2831.