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docs: document runtime non-null fields and add trade-off warnings

Following review feedback on the MarkNonNull feature, add guardrails so the
runtime null-propagation behavior is adopted transparently rather than as
hidden server logic.

- Expand the MarkNonNull godoc with an explicit trade-off warning: altering
  null-propagation at runtime makes the response shape depend on server logic,
  is not introspectable, and breaks static client tooling. Recommend driving
  it from a client-supplied executable directive and using it only when you
  control both client and server.

- Add a recipe (docs/content/recipes/dynamic-required-fields.md) showing the
  recommended pattern: declaring and registering an executable
  @priority(value: REQUIRED) directive, implementing a handler that calls
  MarkNonNull from the directive argument, before/after example queries and
  responses, when not to use it, how it works under the hood, the AroundFields
  alternative, and a note on introspection as future work.