Fix simulation of stateless models by supplying explicit saveat (#32)

For models with no ODE unknowns the adaptive solver takes no internal
steps, leaving sol.t empty and making observed-variable evaluation
impossible. When the system has no unknowns, fall back to 500 evenly
spaced save points across the experiment time span so that observed
variables can be interpolated normally.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: AnHeuermann

src/simulate.jl

@@ -39,7 +39,14 @@ function run_simulate(ode_prob, model_dir::String,
         # to the log file so they don't clutter stdout.
         sol = Logging.with_logger(logger) do
             # Overwrite saveat, always use dense output.
-            solve(ode_prob, Rodas5P(); saveat = Float64[], dense = true)
+            # For stateless models (no unknowns) the adaptive solver takes no
+            # internal steps and sol.t would be empty with saveat=[].
+            # Supply explicit time points so observed variables can be evaluated.
+            sys    = ode_prob.f.sys
+            saveat = isempty(ModelingToolkit.unknowns(sys)) ?
+                         collect(range(ode_prob.tspan[1], ode_prob.tspan[end]; length = 500)) :
+                         Float64[]
+            solve(ode_prob, Rodas5P(); saveat = saveat, dense = true)
         end
         sim_time = time() - t0
         if sol.retcode == ReturnCode.Success