Add Oregonator example with reference verification and work-precision output

[Copilot]

Adds an Oregonator IVP example based on the University of Bari test set, using the published F77 system, initial state, interval, and reference solution at t = 360. The example verifies the terminal state against the reference, then emits work-precision artifacts for tolerance sweeps.

• New Oregonator example

  • adds example/oregonator.f90
  • implements the Oregonator RHS and exact Jacobian from the supplied test problem
  • uses y0 = [1, 2, 3] on t ∈ [0, 360]

• Reference-solution verification

  • checks the computed solution at t = 360 against the provided reference state
  • fails fast if the relative error exceeds a tight verification threshold before generating plot data

• Work-precision artifacts

  • sweeps solver tolerances across multiple runs
  • records error vs. work metrics from solver statistics
  • writes:
    • oregonator_work_precision.csv
    • oregonator_work_precision.svg

• Example registration and docs

  • registers oregonator in the example CMake target list
  • updates the README example list and documents the generated output files

Example usage:

call stiff3(n, fun, x0, y, x1, jac, h0, eps, w, stats=stats)

Example generated data schema:

eps,max_relative_error,nfev,njev,nlu,nsol,nacc,nrej
1.0e-4,...
1.0e-5,...
...