Align core attribute documentation with ADaM IG 1.3 and SDTM standards

[Copilot]

The documentation incorrectly listed "Expected" and "Conditionally Expected" as valid ADaM core values. Per ADaM IG 1.3 pg 14, ADaM only uses: Required, Conditionally Required, and Permissible.

Changes

• README.Rmd/README.md: Updated core attribute description in ds_vars table to specify:
  • ADaM IG 1.3: "Required", "Conditionally Required", "Permissible"
  • SDTM: "Required", "Expected", "Permissible"
• Building_Specification_Readers.Rmd: Added standard-specific core values

Scope

Validation logic in R/validators.R unchanged to avoid breaking existing metadata that may use the broader set of values.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Documentation: core in Readme</issue_title>
<issue_description>

In ADaM IG, core can be Required, Conditionally required or Permissible (ADAM IG 1.3 pg 14)

SDTM IG 3.4 page 22

It is not clear what core values are allowed. Furthermore, in CDISC library, only Req, Cond Perm is used for ADaM and SDTM Req, Exp and Perm. Are these values also acceptable for Core?</issue_description>

<agent_instructions>I need to update the information in the Readme to be more aligned with ADaM IG screenshot provided in the issue</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Documentation: core in Readme #80

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