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/README.md

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+# Snap-through of a shallow arch by arc length
+
+A shallow sinusoidal arch y₀ = a sin(πx/L), L = 100, depth 1, unit width (40 × 2 × 1 hex20,
+E = 10⁴, ν = 0), pinned at mid-depth at both ends, under a sinusoidal dead load q₀ sin(πx/L)
+applied per element on its top face, in a nonlinear static step with arc-length control. With
+a/r = 3.5 the symmetric path has two limit points and the antisymmetric mode does not bifurcate
+before them. The load is scaled to the closed form's upper limit load, so the path peaks at load
+factor 1; the step snaps through, passes the lower limit point and stops at a midspan deflection
+of 2.2 a. Measured: both limit points' load factors (from the change of the tangent's inertia)
+and the one negative eigenvalue between them.