Add wasm leak detector & fix leak

reconcile-js/src/wasm-leak-detector.ts

@@ -0,0 +1,63 @@
+/**
+ * Test utility for detecting WASM memory leaks.
+ *
+ * wasm-bindgen registers every JS-side object with a `FinalizationRegistry`.
+ * This detector patches `FinalizationRegistry.prototype.register` to collect
+ * references to all WASM objects. After each test, {@link checkForWasmLeaks}
+ * inspects `__wbg_ptr` on every tracked object - a non-zero pointer means
+ * `.free()` was never called, i.e. a leak.
+ *
+ * Install once (before any WASM calls) and call {@link checkForWasmLeaks}
+ * in an `afterEach` hook.
+ */
+
+let trackedObjects: object[] = [];
+let originalRegister: Function | null = null;
+
+interface WasmBindgenObject {
+  __wbg_ptr: number;
+  constructor: { name?: string };
+}
+
+function isWasmBindgenObject(target: unknown): target is WasmBindgenObject {
+  return (
+    target !== null &&
+    typeof target === 'object' &&
+    '__wbg_ptr' in (target as Record<string, unknown>)
+  );
+}
+
+/**
+ * Patches `FinalizationRegistry.prototype.register` to track all wasm-bindgen
+ * objects. Safe to call multiple times (idempotent).
+ */
+export function installWasmLeakDetector(): void {
+  if (originalRegister) return;
+
+  originalRegister = FinalizationRegistry.prototype.register;
+
+  FinalizationRegistry.prototype.register = function (
+    target: object,
+    heldValue: unknown,
+    unregisterToken?: object
+  ) {
+    if (isWasmBindgenObject(target)) {
+      trackedObjects.push(target);
+    }
+    return originalRegister!.call(this, target, heldValue, unregisterToken);
+  };
+}
+
+/**
+ * Returns any tracked WASM objects whose `__wbg_ptr` is still non-zero
+ * (i.e. `.free()` was never called). Clears the tracked set afterwards.
+ */
+export function checkForWasmLeaks(): string[] {
+  const leaks = trackedObjects
+    .filter(isWasmBindgenObject)
+    .filter((obj) => obj.__wbg_ptr !== 0)
+    .map((obj) => `${obj.constructor?.name ?? 'Unknown'} (ptr=${obj.__wbg_ptr})`);
+
+  trackedObjects = [];
+  return leaks;
+}