refactor(ui): use asEntityId helper at piece-id boundaries

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

packages/rete/src/index.ts

@@ -6,7 +6,7 @@ export type {
   SchemaKind,
   DefineSchemaOptions,
 } from "./schema.js";
-export { defineSchema, fact } from "./schema.js";
+export { asEntityId, defineSchema, fact } from "./schema.js";
 
 export type { AttrKey, FactValue } from "./wm.js";
 export { WorkingMemory } from "./wm.js";

packages/rete/src/schema.ts

@@ -24,6 +24,16 @@
  */
 export type EntityId = number & { readonly __brand: "EntityId" };
 
+/**
+ * Coerce a raw number to the branded EntityId type. Use ONLY at trust
+ * boundaries where you've already established that `n` came from
+ * `Session.nextId()` or a fact's `id` field (e.g. UI code that carries
+ * a piece's id as `number` because React prop serialization doesn't
+ * preserve branding). Prefer passing `EntityId` through end-to-end when
+ * possible; this helper is the single legitimate cast site.
+ */
+export const asEntityId = (n: number): EntityId => n as EntityId;
+
 /**
  * Shape of a schema type parameter.
  *