Move mesh inertia specification from global <compiler> to <asset/mesh>.

Three options:
- Convex: Use convex hull
- Exact: Use exact geometry (requires watertight mesh)
- Legacy: Legacy approximation (deprecated)

Fixes #2152

PiperOrigin-RevId: 688949900
Change-Id: I4537f343db917195c6fd58d517b189d5ee3be1cd

src/user/user_init.c

@@ -246,6 +246,7 @@ void mjs_defaultMesh(mjsMesh* mesh) {
   mesh->refquat[0] = 1;
   mesh->scale[0] = mesh->scale[1] = mesh->scale[2] = 1;
   mesh->maxhullvert = -1;
+  mesh->inertia = mjINERTIA_LEGACY;
 }