fem, solver: thermal strain in nonlinear static steps

Temperatures now strain the material in nonlinear steps instead of being
refused. At finite strain the deformation gradient splits multiplicatively,
F = F_m F_theta with F_theta = diag(1 + alpha_i dT) at each point: the material
sees F_m and the loops run on the thermally expanded volume J_theta dV0 (mixed
elements project det F_m), so the tangent stays consistent. At small strain the
split is additive, and elements on their linear stiffness take the equivalent
nodal forces as a dead load. The step's temperatures are its temperature loads,
or the steady conduction of its temperature conditions and heat loads (one-way
coupling, as the linear coupled step); both at once are refused. They ramp with
the load factor from each material's reference temperature. Johnson-Cook
plasticity softens at the point's temperature. Verified: free expansion and a
fully constrained neo-Hookean block at 20 % thermal stretch, full and hybrid
(exact); a bar held at its ends heated past yield (exact); the linear limit and
the conducted temperatures against the linear solvers; tangents by central
differences with a temperature field.

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

crates/fem/src/plasticity.rs

@@ -229,6 +229,11 @@ impl J2 {
         Ok(J2 { shear, bulk, flow, damage, heating })
     }
 
+    /// Whether the flow stress depends on the temperature (Johnson–Cook softening).
+    pub fn is_temperature_dependent(&self) -> bool {
+        matches!(self.flow, Some(Flow::JohnsonCook { .. }))
+    }
+
     /// The temperature a point starts at.
     pub fn initial_temperature(&self) -> f64 {
         match self.flow {