fix: introduce NativeBeaconStateView and simplify bindings

packages/state-transition/src/stateView/interface.ts

@@ -278,6 +278,35 @@ export type IBeaconStateViewLatestFork = Omit<
   latestExecutionPayloadHeader: ExecutionPayloadHeader;
   payloadBlockNumber: number;
 };
+
+/**
+ * Contract a BeaconStateView backing implementation must satisfy.
+ *
+ * Differs from `IBeaconStateViewLatestFork` in two ways:
+ * - `executionPayloadAvailability` is a raw `{uint8Array, bitLen}` POJO — a
+ *   native (`.node`) binding cannot construct a `BitArray` across FFI.
+ *   `NativeBeaconStateView` lifts it back to `BitArray` for beacon-node.
+ * - Methods that produce another view (`stateTransition`, `processSlots`,
+ *   `loadOtherState`, `withParentPayloadApplied`) return `IBeaconStateViewNative`
+ *   so callers can re-wrap without an `as unknown` cast. Param lists are reused
+ *   via `Parameters<...>` to avoid duplicating signatures.
+ *
+ * The TS-side `BeaconStateView` also structurally satisfies this contract since
+ * `BitArray` exposes `uint8Array` and `bitLen`.
+ */
+export type IBeaconStateViewNative = Omit<
+  IBeaconStateViewLatestFork,
+  "executionPayloadAvailability" | "loadOtherState" | "stateTransition" | "processSlots" | "withParentPayloadApplied"
+> & {
+  executionPayloadAvailability: {uint8Array: Uint8Array; bitLen: number};
+  loadOtherState(...args: Parameters<IBeaconStateViewLatestFork["loadOtherState"]>): IBeaconStateViewNative;
+  stateTransition(...args: Parameters<IBeaconStateViewLatestFork["stateTransition"]>): IBeaconStateViewNative;
+  processSlots(...args: Parameters<IBeaconStateViewLatestFork["processSlots"]>): IBeaconStateViewNative;
+  withParentPayloadApplied(
+    ...args: Parameters<IBeaconStateViewLatestFork["withParentPayloadApplied"]>
+  ): IBeaconStateViewNative;
+};
+
 export function isStatePostAltair(state: IBeaconStateView): state is IBeaconStateViewAltair {
   return isForkPostAltair(state.forkName);
 }