docs: revert cross-field validation to and! with simple comparison; add note for validated-field checks

[Copilot]

The README's Global Validation example was updated to use let! with post-validated field values, diverging from the intended simple, pre-validated comparison pattern. Reverts that change and adds a forward reference for readers who need the more advanced pattern.

Changes

• Reverted cross-field block from let! _ = (sequential, post-validated) back to and! _ = (parallel, pre-validated) using withValue vm with a direct field comparison:

  and! _ = validation {
      withValue vm
      disputeWithFact NameMatchesUsername (fun a -> a.Name <> a.Username)
      qed
  }

• Added note after the example pointing readers to the Cross-Field Validation section for cases where checks must operate on individually-validated values.

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