Pure, immutable ParametrizedFunction #568

Replace the stateful parameter contract of ParametrizedFunction with pure evaluation:

• function(data, parameters): parameter values are merged with the construction-time defaults per call and are never sticky.
• with_parameters(): returns a new function with updated defaults (for post-fit plotting, data generation, etc.). The compiled backend function is shared, so this is cheap.
• update_parameters() is removed and ParametrizedBackendFunction becomes immutable (frozen).

This makes functions thread-safe and free of hidden side effects: estimators no longer mutate the user's function on every call. Estimator internals switch to the pure call and the documentation notebooks migrate to the new idioms in the same PR.