oracle: quantities of a generated mesh can be gated by their tolerance alone ("fixture": false)

The bracket's tet mesh is not bit-identical across platforms (#115): its fillet peak is 213.31 MPa
on macOS and 212.42 on Linux CI, which turned the oracle job red after #110. Those two quantities
are no longer frozen; the page names them mesh-dependent instead of printing one platform's value.

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

tests/oracle/filleted-bracket-fillet-stress/fixture.json

@@ -1,12 +1,8 @@
 {
   "note": "FieldCraft's accepted results for this case, written by `cargo xtask oracle --bless`. A result outside its band fails CI.",
   "values": {
-    "fillet_peak": {
-      "band": 2.1330693756948646e-7,
-      "value": 213.30693756948645
-    },
     "reaction": {
-      "band": 2.0000000000001007e-6,
+      "band": 2.0000000000001007e-06,
       "value": -2000.0000000001007
     }
   }