fix(runtime): built-in prototype methods are non-enumerable (spec descriptor)

[proggeramlug]

Summary

Built-in prototype methods (Array/String/Number/Object/TypedArray/… .prototype.*) were installed via the ordinary field-set path, which defaults to { writable: true, enumerable: true, configurable: true }. Per spec they are non-enumerable, so Perry disagreed with Node on all three observation paths:

Object.getOwnPropertyDescriptor(Array.prototype, "map").enumerable  // Perry: true,  Node: false
for (const k in Array.prototype) { /* ... */ }                      // Perry: 42 methods, Node: 0
Object.keys(Array.prototype)                                        // Perry: listed them, Node: []

This fails Test262's pervasive verifyProperty checks (every built-in method test verifies the property is non-enumerable, via both getOwnPropertyDescriptor and a for-in scan).

Fix

Record a non-enumerable data-property descriptor at install time, via a new set_builtin_property_attrs helper. Like the existing #2060 built-in accessor path, it inserts into PROPERTY_DESCRIPTORS without flipping the process-wide GLOBAL_DESCRIPTORS_IN_USE hot-path gate.

This works because getOwnPropertyDescriptor, Object.keys, and for-in (which lowers to Object.keys) each read PROPERTY_DESCRIPTORS per-object and unconditionally — so they now observe enumerable:false — while the object get/set hot path keeps skipping the descriptor table for every program (no perf regression).

Testing

Now matches Node exactly:

for (k in Array.prototype) → 0 keys
Object.getOwnPropertyDescriptor(Array.prototype,"map") → { writable:true, enumerable:false, configurable:true }

User-object for-in / Object.keys unaffected ({a,b,c} → ['a','b','c']). cargo fmt clean.

Scope

Foundational spec-correctness for the reflective layer. On its own it moves the Test262 conformance % only marginally (each verifyProperty test also gates on writable-via-write, configurable-via-delete, exact value, and .name/.length own-properties — all of which must pass together), but the non-enumerable descriptor is a prerequisite that every built-in-method conformance test needs.

Version bump + CHANGELOG entry omitted — please fold in at merge.