Fmi2 directional derivatives

[davidhjp01]

Add FMI 2 directional derivative support

Summary

Adds validated FMI 2.0 directional-derivative support to
fmi::v2::slave_instance, lifecycle-aware FMU state handling, deterministic
fixture coverage, and the dependency/CI updates required to build and test it.

Changes

• Add provides_directional_derivatives() and
  get_directional_derivative().

• Read and cache directional-derivative metadata from
  modelDescription.xml.

• Validate lifecycle, value-reference type, reference validity, and seed and
  sensitivity sizes before calling the FMU.

• Compute the Jacobian-vector product:

  sensitivity = d(unknowns) / d(knowns) * seed
  

• Accept FMI OK and Warning; surface Discard, Error, Fatal, and
  Pending as errors.

• Preserve caller output when the FMU call fails.

• Track instantiated, initialization, step, and terminated states.

• Serialize lifecycle state in export scheme 1 while retaining safe scheme 0
  compatibility.

• Add cleanup and ownership checks for imported FMU state.

Test coverage

• Add a deterministic FMI 2 Co-Simulation fixture with three Real inputs, two
  Real outputs, a fixed 2-by-3 Jacobian, configurable statuses, a call counter,
  and a non-Real reference.
• Extend fmi_v2_fmu_unittest for lifecycle behavior, expected derivatives,
  reordered references, invalid inputs, status handling, output preservation,
  and unsupported capabilities.
• Build the pinned Reference-FMUs v0.0.41 and package the fixture as
  Fmi2Derivatives.fmu; expose them through REFERENCE_FMU_V2_DIR.

Dependency and CI updates

• Upgrade FMI Library to ~3.0.4 and proxyFMU to 0.4.3@osp/testing.

[kyllingstad]

Thanks! Great to finally have some more FMI 2.0 support… :D I have a few comments, mostly on form rather than substance.

[kyllingstad]

I think this error message is misleading. Getting directional derivatives is not a "lifecycle operation", it is an operation that may be invalid at the current state in the FMU's lifecycle. I'm also not sure whether errc::model_error is the right error code for this. Maybe we need a new one, e.g. errc::invalid_operation or something. (It wouldn't be unique to this particular situation; there are many cases in the FMI spec where certain operations are prohibited at a certain lifecycle stage. Could also be used for the "invalid (un)known" errors further down in this function.)

[davidhjp01]

Updated to throw invalid_operation and text

[kyllingstad]

Almost there now. :)

[kyllingstad]

This should not be reverted, IMO. It is a new scheme version, my point was just that shouldn't worry about maintaining compatibility with the previous one.

[davidhjp01]

Made it back to 1

[kyllingstad]

Nice, thanks for doing this!