oracle: 2D cases (Kirsch plane stress, Lamé plane strain and axisymmetric, rotating disc and cylinder, NAFEMS LE1 with quad8) and J2 uniaxial explicit

The pending 2D LE1 case becomes a real one; the plasticity cases stay
pending with what they still need (static J2 for the collapse pressure,
the transcribed measurements for the Taylor bar).

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

tests/oracle/kirsch-plate-plane-stress/fixture.json

@@ -0,0 +1,13 @@
+{
+  "note": "ModelCraft's accepted results for this case, written by `cargo xtask oracle --bless`. A result outside its band fails CI.",
+  "values": {
+    "kt": {
+      "band": 3.0028789592374985e-9,
+      "value": 3.0028789592374983
+    },
+    "side": {
+      "band": 1.0018737796236792e-9,
+      "value": -1.0018737796236792
+    }
+  }
+}