Unify RHS and SOLOUT user interrupts under a single idid exit path

[Copilot]

This change makes user-driven early termination behave consistently whether it originates in SOLOUT or during RHS evaluation. Both callback paths now return through the same solver status (idid = -2) while preserving the last accepted integration state.

• Solver API

  • Extend rhs_sub with an ires in/out flag.
  • Treat ires < 0 the same as irtrn < 0 from solout: return idid = -2.

• Core integration flow

  • Thread RHS interrupt handling through the main step logic and the internal sirk3 stage evaluation.
  • Restore the last accepted (x, y) state when an RHS interrupt occurs mid-step, so callback exits remain restart-safe and consistent with existing solout semantics.

• Docs and call sites

  • Update README and all examples/tests to the new RHS callback signature.
  • Clarify that idid = -2 now represents a user interrupt from either callback surface.

• Regression coverage

  • Extend ode_hmax to cover RHS-triggered interruption and assert that the solver returns the prior accepted state rather than a partially advanced trial state.

Example updated callback shape:

subroutine fun(n, y, f, ires)
  integer, intent(in) :: n
  real(wp), intent(in) :: y(n)
  real(wp), intent(inout) :: f(n)
  integer, intent(inout) :: ires

  f(1) = -y(1)
  if (should_stop(y)) ires = -1
end subroutine