verification: snap-through of a shallow arch by arc length

arc-shallow-arch: a sinusoidal arch of 40 × 2 × 1 hex20 under a sinusoidal dead load scaled to
the one-mode closed form's upper limit load: the upper limit point at 0.992 (−0.8 %), the lower
at 0.219 (0.8 % of the peak load) and exactly one negative eigenvalue between them. ROADMAP and
parity updated, docs/oracle.md regenerated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

tests/oracle/arc-shallow-arch/fixture.json

@@ -0,0 +1,17 @@
+{
+  "note": "FieldCraft's accepted results for this case, written by `cargo xtask oracle --bless`. A result outside its band fails CI.",
+  "values": {
+    "negative": {
+      "band": 1e-9,
+      "value": 1.0
+    },
+    "peak": {
+      "band": 1e-9,
+      "value": 0.9918292211513776
+    },
+    "valley": {
+      "band": 0.000020000218947968203,
+      "value": 0.21894796820214898
+    }
+  }
+}