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

test/user/testdata/fitmesh_inertiabox.xml

@@ -1,8 +1,8 @@
 <mujoco>
-  <compiler fitaabb="false" exactmeshinertia="true"/>
+  <compiler fitaabb="false"/>
 
   <asset>
-    <mesh file="stanford_bunny.obj"/>
+    <mesh file="stanford_bunny.obj" inertia="exact"/>
   </asset>
 
   <worldbody>

test/user/testdata/inertia_compare.xml

@@ -0,0 +1,24 @@
+<mujoco>
+  <asset>
+    <mesh name="incomplete_convex" inertia="convex"
+          vertex="0 0 0  1 0 0  0 1 0  0 0 1"
+          face="2 0 3  0 1 3  1 2 3" />
+    <mesh name="incomplete"
+          vertex="0 0 0  1 0 0  0 1 0  0 0 1"
+          face="2 0 3  0 1 3  1 2 3"/>
+    <mesh name="complete"
+          vertex="0 0 0  1 0 0  0 1 0  0 0 1"
+          face="2 0 3  0 1 3  1 2 3  1 0 2"/>
+  </asset>
+  <worldbody>
+    <body>
+      <geom type="mesh" mesh="incomplete_convex"/>
+    </body>
+    <body>
+      <geom type="mesh" mesh="incomplete"/>
+    </body>
+    <body>
+      <geom type="mesh" mesh="complete"/>
+    </body>
+  </worldbody>
+</mujoco>

test/user/testdata/inertia_concave.xml

@@ -1,9 +1,11 @@
 <mujoco>
-  <compiler exactmeshinertia="true"/>
+  <default>
+    <mesh scale="10 10 10" inertia="exact"/>
+  </default>
   <asset>
-    <mesh file="cube_cup.obj" scale="10 10 10"/>
-    <mesh file="cube_cup_quad.obj" scale="10 10 10"/>
-    <mesh file="cube_cup_hi.obj" scale="10 10 10"/>
+    <mesh file="cube_cup.obj"/>
+    <mesh file="cube_cup_quad.obj"/>
+    <mesh file="cube_cup_hi.obj"/>
   </asset>
   <worldbody>
     <light directional="true" diffuse=".6 .6 .6" specular="0.2 0.2 0.2" pos="0 0 4" dir="0 0 -1"/>

test/user/testdata/inertia_convex.xml

@@ -1,7 +1,7 @@
 <mujoco>
   <asset>
     <mesh file="cube.obj"/>
-    <mesh name="inline_cube"
+    <mesh name="inline_cube" inertia="convex"
         vertex="
           -.5 -.5 -.5
           .5 -.5 -.5