oracle: an ill-conditioned quantity gets a wider fixture band; Scordelis-Lo re-blessed

Main's oracle job failed at 8cba3a7: scordelis-lo-roof-solid u_z moved from
-0.3014029226397275 (blessed before #50) beyond its 1e-9 band. The case is a
thin shell (R/t = 100) of one layer of hex20R solids, ill-conditioned, so
rounding alone moves u_z by about 1e-9 relative: on macOS #50's multithreaded
supernodal factorisation moved it by 8.2e-10 (to -0.3014029223929), #52's
interpolation order by a further 1.3e-10 (to -0.3014029223541), and Linux
CI by slightly more, past the band. #52 kept the lattice geometry the same.
The result itself is unchanged at the 1e-9 level (0.33% from the
reference).

A quantity may now set `fixture_relative` (with a required
`fixture_reason`) for a wider fixture band; Scordelis-Lo uses 1e-7 and is
re-blessed. Moved-fixture messages print full precision.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

tests/oracle/scordelis-lo-roof-solid/expected.json

@@ -24,7 +24,9 @@
    "tolerance": {
     "relative": 0.02
    },
-   "reason": "MacNeal and Harder grade an element A within 2% of the theoretical value on this prescribed coarse mesh (p. 14); MacNeal and Harder normalise by 0.3024 (the deep-shell value quoted in their [5] is 0.3086), and a solid model of R/t = 100 differs from shell theory by O(t/R)"
+   "reason": "MacNeal and Harder grade an element A within 2% of the theoretical value on this prescribed coarse mesh (p. 14); MacNeal and Harder normalise by 0.3024 (the deep-shell value quoted in their [5] is 0.3086), and a solid model of R/t = 100 differs from shell theory by O(t/R)",
+   "fixture_relative": 1e-7,
+   "fixture_reason": "a thin shell (R/t = 100) of one layer of solids is ill-conditioned: rounding moves u_z by about 1e-9 relative between equivalent orderings of the arithmetic (8e-10 when the solver's factorisation changed in #50, 1e-10 from the interpolation order in #52, more between platforms); 1e-7 still catches any real change"
   }
  ]
 }

tests/oracle/scordelis-lo-roof-solid/fixture.json

@@ -2,8 +2,8 @@
   "note": "ModelCraft's accepted results for this case, written by `cargo xtask oracle --bless`. A result outside its band fails CI.",
   "values": {
     "uz": {
-      "band": 3.024e-10,
-      "value": -0.3014029226397275
+      "band": 3.024e-8,
+      "value": -0.30140292235414196
     }
   }
 }