refactor(engine): drop pointless FactValue double-cast in PresetState.set

FactValue is `unknown` in @paratype/rete, so `value as unknown as FactValue`
was `unknown` → `unknown` → `unknown` — zero type narrowing, just noise.
The stale comment also claimed FactValue was `string | number | boolean | null`
(it isn't, and hasn't been for a while).

Pass value directly; update the comment to reflect actual serialization
responsibility (caller ensures JSON-round-trippable for event-log replay).

packages/chess/src/engine.ts

@@ -259,13 +259,14 @@ class PresetStateImpl<T extends Record<string, unknown>>
   }
 
   set<K extends keyof T>(key: K, value: T[K]): void {
-    // The rete session validates that `value` is a legal FactValue
-    // (string | number | boolean | null). Passing objects here will
-    // throw — a deliberate constraint to keep facts serialization-safe.
+    // `FactValue` is `unknown` in the rete package, so any `T[K]`
+    // assigns directly. Runtime serialization guarantees (the value
+    // round-trips through JSON in event-log replay) are the caller's
+    // responsibility.
     this.#session.insert(
       PRESET_STATE_ENTITY,
       this.#attrOf(key as string),
-      value as unknown as import("@paratype/rete").FactValue,
+      value,
     );
   }