Fix null handling in controllable signal

packages/core/src/primitives/create-controllable-signal.ts

@@ -3,7 +3,7 @@ import { type Accessor, createSignal } from "solid-js";
 export interface CreateControllableSignalOptions<T> {
   /** Returns the controlled value, or undefined if uncontrolled. */
   value: Accessor<T | undefined>;
-  /** Default value used when uncontrolled. */
+  /** Default value used when uncontrolled. Only read once at creation time — not reactive after mount. */
   defaultValue: Accessor<T>;
   /** Called whenever the value changes (both modes). */
   onChange?: (value: T) => void;
@@ -21,12 +21,14 @@ export function createControllableSignal<T>(
 
   const get: Accessor<T> = () => {
     const controlled = options.value();
-    return controlled !== undefined ? controlled : internalValue();
+    if (controlled !== undefined) return controlled;
+    return internalValue();
   };
 
   const set = (value: T) => {
     const isControlled = options.value() !== undefined;
     if (!isControlled) {
+      // Use functional form so SolidJS does not interpret T-as-function as an updater.
       setInternalValue(() => value);
     }
     options.onChange?.(value);