Fix segfault in FuseStatic when body contains cameras or lights

FuseStatic deletes static (joint-less) bodies and reassigns their
children to the parent body. Geoms and sites were reassigned, but
cameras and lights were not, leaving dangling body pointers that
crash in mjCCamera::Compile() / mjCLight::Compile().

Add camera and light reassignment in FuseStatic and FuseReindex.

src/user/user_model.cc

@@ -4138,6 +4138,8 @@ void mjCModel::FuseReindex(mjCBody* body) {
   makelistid(joints_, body->joints);
   makelistid(geoms_, body->geoms);
   makelistid(sites_, body->sites);
+  makelistid(cameras_, body->cameras);
+  makelistid(lights_, body->lights);
 
   // process children recursively
   for (int i=0; i < body->bodies.size(); i++) {
@@ -4265,10 +4267,25 @@ void mjCModel::FuseStatic(void) {
       mju_error("Internal error: FuseStatic: body not found");
     }
 
-    //------------- assign geoms and sites to parent, change frames
+    //------------- assign geoms, sites, cameras, lights to parent, change frames
 
     ReassignChild(par->geoms, body->geoms, par, body);
     ReassignChild(par->sites, body->sites, par, body);
+    ReassignChild(par->cameras, body->cameras, par, body);
+
+    // lights have dir instead of quat, so handle separately
+    for (int j=0; j < body->lights.size(); j++) {
+      body->lights[j]->body = par;
+      par->lights.push_back(body->lights[j]);
+
+      // transform pos into parent frame
+      double qunit[4] = {1, 0, 0, 0};
+      changeframe(body->lights[j]->pos, qunit, body->pos, body->quat);
+
+      // rotate dir into parent frame
+      mjuu_rotVecQuat(body->lights[j]->dir, body->lights[j]->dir, body->quat);
+    }
+    body->lights.clear();
 
     //------------- remove from global body list, reduce global counts
 
@@ -4300,6 +4317,8 @@ void mjCModel::FuseStatic(void) {
     joints_.clear();
     geoms_.clear();
     sites_.clear();
+    cameras_.clear();
+    lights_.clear();
     FuseReindex(bodies_[0]);
 
     // recompute parent contype, conaffinity, and margin

test/user/user_model_test.cc

@@ -668,6 +668,53 @@ TEST_F(FuseStaticTest, FuseStaticForceSensorReferencedBody) {
   mj_deleteModel(m);
 }
 
+TEST_F(FuseStaticTest, FuseStaticCameraInBody) {
+  static constexpr char xml[] = R"(
+  <mujoco>
+    <compiler fusestatic="true"/>
+    <worldbody>
+      <body>
+        <joint axis="1 0 0"/>
+        <geom size="0.5"/>
+        <body pos="1 0 0">
+          <site name="site1"/>
+          <camera name="cam1"/>
+        </body>
+      </body>
+    </worldbody>
+  </mujoco>
+  )";
+  std::array<char, 1024> error;
+  mjModel* m = LoadModelFromString(xml, error.data(), error.size());
+  ASSERT_THAT(m, NotNull()) << error.data();
+  EXPECT_EQ(m->nbody, 2) << "Static body should be fused";
+  EXPECT_EQ(m->ncam, 1);
+  mj_deleteModel(m);
+}
+
+TEST_F(FuseStaticTest, FuseStaticLightInBody) {
+  static constexpr char xml[] = R"(
+  <mujoco>
+    <compiler fusestatic="true"/>
+    <worldbody>
+      <body>
+        <joint axis="1 0 0"/>
+        <geom size="0.5"/>
+        <body pos="1 0 0">
+          <light name="light1" dir="0 0 -1"/>
+        </body>
+      </body>
+    </worldbody>
+  </mujoco>
+  )";
+  std::array<char, 1024> error;
+  mjModel* m = LoadModelFromString(xml, error.data(), error.size());
+  ASSERT_THAT(m, NotNull()) << error.data();
+  EXPECT_EQ(m->nbody, 2) << "Static body should be fused";
+  EXPECT_EQ(m->nlight, 1);
+  mj_deleteModel(m);
+}
+
 // ------------- test discardvisual --------------------------------------------
 
 using DiscardVisualTest = MujocoTest;