Add policies for global attribute conflict resolution upon attach

For example when loading parent_merge.xml:

```
WARNING: Attach conflict when attaching 'child' to 'parent_merge', policy is 'merge'
timestep: parent has 0.005, child has 0.002, taking the minimum
iterations: parent has 50, child has 100, taking the maximum
flag 'Damper': added from child
```

When loading parent_error.xml:

```
XML Error: Attach conflict when attaching 'child' to 'parent_error', policy is 'error'
timestep: parent has 0.005, child has 0.002
iterations: parent has 50, child has 100
Element 'attach', line 10
```
PiperOrigin-RevId: 933620810
Change-Id: Ib477863b5ef763474d27fb4be5a4148be1d5d500

doc/includes/references.h

@@ -1549,6 +1549,11 @@ typedef enum mjtOrientation_ {     // type of orientation specifier
   mjORIENTATION_ZAXIS,             // z axis (minimal rotation)
   mjORIENTATION_EULER,             // Euler angles
 } mjtOrientation;
+typedef enum mjtConflict_ {  // conflict resolution for attach
+  mjCONFLICT_WARNING = 0,    // keep parent, warn on conflict
+  mjCONFLICT_MERGE,          // merge: min/max/error per field
+  mjCONFLICT_ERROR,          // error on any conflict
+} mjtConflict;
 typedef enum mjtCTimer_ {          // compiler timing categories
   // top-level timers (wall-clock)
   mjCTIMER_TOTAL = 0,              // total compile time
@@ -1585,6 +1590,7 @@ typedef struct mjsCompiler_ {      // compiler options
   int inertiagrouprange[2];        // range of geom groups used to compute inertia
   mjtByte saveinertial;            // save explicit inertial clause for all bodies to XML
   int alignfree;                   // align free joints with inertial frame
+  int conflict;  // conflict resolution for attach (mjtConflict)
   mjLROpt LRopt;                   // options for lengthrange computation
   mjString* meshdir;               // mesh and hfield directory
   mjString* texturedir;            // texture directory