Per-engine realm isolation

Date: 2026-04-25 Area: runtime Pull Request: #403

Per-engine realm for built-in prototype isolation. TGocciaRealm (Goccia.Realm.pas) owns the mutable intrinsic state — Array.prototype, Object.prototype, Map.prototype, every error prototype, every Temporal prototype, etc. Each TGocciaEngine constructs its own realm and frees it in Destroy, which unpins every prototype the realm owns; the next engine on the same worker thread sees pristine intrinsics. Replaces the unit-level threadvar caches that previously survived engine teardown and the prototypeIsolation.js harness that tried to undo mutations from JS (and could not reverse non-configurable property additions). Two slot kinds: TGocciaRealmSlotId for TGCManagedObject prototypes (pinned in SetSlot, unpinned at tear-down), and TGocciaRealmOwnedSlotId for plain-TObject helpers like TGocciaSharedPrototype (Free-d at tear-down before pin release so destructors can still unpin owned objects). core-patterns.md § Realm Ownership & Slot Registration. embedding.md § Engine Lifecycle & Realm Isolation.