Add compiler timing diagnostics to mjsCompiler, printed by compile.cc

For example, `compile mujoco_menagerie/robotis_op3/scene.xml` now outputs

```
Compile 1 (cold cache):
  total:      317.2 ms
  assets:     284.8 ms (wall clock)
    load:     616.1 ms
    hull:      26.4 ms
    poly:     137.8 ms
    inert:    177.8 ms
    bvh:      568.2 ms
    octr:       1.6 ms
    tex:       25.3 ms
  other:       32.4 ms

Compile 2 (warm cache):
  total:       79.9 ms
  assets:      54.5 ms (wall clock)
    load:     888.5 ms
    hull:       0.0 ms
    poly:       0.0 ms
    inert:      0.0 ms
    bvh:        0.0 ms
    octr:       0.0 ms
    tex:       21.4 ms
  other:       25.3 ms
```

PiperOrigin-RevId: 917850214
Change-Id: Iaec86230bec0faf2e47820e20cbff61de5b2621e

doc/changelog.rst

@@ -17,6 +17,10 @@ General
   release, now uses a fixed seed. The previous implementation seeded with ``mjData.time``, which introduced subtle yet
   undesirable time dependence.
 - Flexes are now allowed to sleep, with the exception of completely passive (constraint-free) flexes.
+- Added compiler timing diagnostics via the new :ref:`mjtCTimer` enum and the :ref:`mjs_getTimer` C API. After
+  :ref:`mj_compile`, per-category timings (total, assets, mesh loading, convex hull, normals, inertia, BVH, octree,
+  textures) are available via ``mjs_getTimer(spec)``. The :ref:`compile<saCompile>` sample prints a detailed timing
+  breakdown when run without an output file.
 
 .. admonition:: Breaking API changes
    :class: attention