refactor(joint): drop underscore from getFixedBody

The underscore claims class privacy, but getFixedBody is part of the
component/system contract (called from JointComponent). Name it plainly
and rely on @ignore to keep it off the public docs/API surface.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: willeastcott

src/framework/components/joint/component.js

@@ -1401,7 +1401,7 @@ class JointComponent extends Component {
         const bodyA = rigidbodyA.body;
 
         // world-pinned joints attach to a shared static body with an identity transform
-        const bodyB = rigidbodyB ? rigidbodyB.body : this.system._getFixedBody();
+        const bodyB = rigidbodyB ? rigidbodyB.body : this.system.getFixedBody();
 
         Debug.call(() => {
             if (rigidbodyA.type !== BODYTYPE_DYNAMIC && (!rigidbodyB || rigidbodyB.type !== BODYTYPE_DYNAMIC)) {

src/framework/components/joint/system.js

@@ -43,7 +43,7 @@ class JointComponentSystem extends ComponentSystem {
     _breakable = new Set();
 
     /**
-     * Shared static Ammo body that world-pinned joints attach to - see {@link _getFixedBody}.
+     * Shared static Ammo body that world-pinned joints attach to - see {@link getFixedBody}.
      *
      * @type {object|null}
      * @private
@@ -82,7 +82,7 @@ class JointComponentSystem extends ComponentSystem {
      * @returns {object} The shared static Ammo body.
      * @ignore
      */
-    _getFixedBody() {
+    getFixedBody() {
         if (!this._fixedBody) {
             const transform = new Ammo.btTransform();
             transform.setIdentity();