Skip flex edge Jacobian if not needed.

When equality constraints and built-in passive forces are not present, edge Jacobians are unused. This gives a 3x speedup in `mj_kinematics` in `floppy_flex` (from 0.2ms/step to 0.06ms/step).

Changed the default options of `floppy_flex` to match those of `floppy` for a fair comparison.

PiperOrigin-RevId: 575850622
Change-Id: I4c618536fd17ef930153bbd1f1da7d9e0e44abbe

src/engine/engine_core_smooth.c

@@ -541,6 +541,11 @@ void mj_flex(const mjModel* m, mjData* d) {
       continue;
     }
 
+    // skip Jacobian if no built-in passive force is needed
+    int skipjacobian = !m->flex_edgeequality[f] &&
+                       !m->flex_edgedamping[f] &&
+                       !m->flex_edgestiffness[f];
+
     // process edges of this flex
     int vbase = m->flex_vertadr[f];
     int ebase = m->flex_edgeadr[f];
@@ -557,6 +562,11 @@ void mj_flex(const mjModel* m, mjData* d) {
       mju_sub3(vec, pos2, pos1);
       d->flexedge_length[ebase+e] = mju_normalize3(vec);
 
+      // skip Jacobian if not needed
+      if (skipjacobian) {
+        continue;
+      }
+
       // sparse edge Jacobian
       if (issparse) {
         // set rowadr