oracle: re-bless the explicit cases after #42

The large-deformation explicit core of #42 (objective stress rates, updated
kinematics) moves the bar wave and rigid-wall impact results by a few 1e-4
relative; both still pass their references (u_end 0.034%, rebound KE
-0.54%). Fixtures updated deliberately for that change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

tests/oracle/bar-rigid-wall-impact/fixture.json

@@ -3,15 +3,15 @@
   "values": {
     "ke": {
       "band": 1e-9,
-      "value": 0.9946927802814477
+      "value": 0.9946334652695279
     },
     "u_back": {
       "band": 1.0000000000000001e-11,
-      "value": 0.009946347263883909
+      "value": 0.009948781428081829
     },
     "u_front": {
       "band": 1e-11,
-      "value": 0.009949683285107395
+      "value": 0.009954136983299318
     }
   }
 }