Harden simulation and reference comparison validation

[AnHeuermann]

Issue

Some models seem to pass simulation but aren't producing any results, e.g. Modelica.Blocks.Examples.BusUsage:

The issue is that the simulation result has no time points at all. Simulation and verification stages should fail in this case.

Changes

• Fail run_simulate when sol.t is empty or the system has no states or observed variables, preventing empty CSVs and downstream panics
• Include observed variables in the simulation CSV output so models without state variables (e.g. BusUsage) still produce usable results
• Fail compare_with_reference (instead of skip) when reference signals are absent from the simulation; write NaN sim/relerr columns for signals with no simulation counterpart
• Validate upfront that all comparisonSignals.txt entries exist in the reference CSV, and error if the simulation time interval does not cover the reference interval
• Extend the per-model status line to show a CMP OK/FAIL phase and signal counts including skipped signals