feat(@probitas/probitas): add custom serializer for IPC step values

Standard JSON.stringify fails on BigInt, circular references, Symbol,
Function, Map, Set, RegExp, TypedArray, and other JavaScript types.
This causes step context values to be lost or corrupted when passed
between processes via IPC.

The custom serializer uses a marker object pattern ($type property)
to preserve type information across the IPC boundary, enabling steps
to receive BigInt, Map, circular references, and other complex values
through ctx.previous and ctx.results.

Handles: BigInt, Symbol, Function, undefined, circular references,
Map, Set, WeakMap, WeakSet, RegExp, Date, Error, TypedArray,
ArrayBuffer, DataView, WeakRef.

Addresses #80

Author: lambdalisue

src/cli/_templates/run_protocol.ts

@@ -18,6 +18,7 @@ import {
   isErrorObject,
   toErrorObject,
 } from "@core/errorutil/error-object";
+import { deserialize, serialize } from "./serializer.ts";
 
 /**
  * Message sent from CLI to subprocess
@@ -163,11 +164,17 @@ export function deserializeError(serialized: ErrorObject): Error {
 }
 
 /**
- * Serialize StepResult error for transmission
+ * Serialize StepResult for transmission
+ *
+ * Handles both error serialization (for failed/skipped) and value serialization
+ * (for passed steps) to ensure all non-JSON-serializable values are converted.
  */
 export function serializeStepResult(result: StepResult): StepResult {
   if (result.status === "passed") {
-    return result;
+    return {
+      ...result,
+      value: serialize(result.value),
+    };
   }
   return {
     ...result,
@@ -193,11 +200,17 @@ export function serializeScenarioResult(
 }
 
 /**
- * Deserialize StepResult error from transmission
+ * Deserialize StepResult from transmission
+ *
+ * Handles both error deserialization (for failed/skipped) and value deserialization
+ * (for passed steps) to restore non-JSON-serializable values.
  */
 export function deserializeStepResult(result: StepResult): StepResult {
   if (result.status === "passed") {
-    return result;
+    return {
+      ...result,
+      value: deserialize(result.value),
+    };
   }
   if (isErrorObject(result.error)) {
     return {