fix: rename PK summary exclusion variables to CDISC names (#1394)

[wangzhengdna-lang]

Issue

Closes #1394

Description

Renames the PK summary exclusion variables to CDISC-compliant names across code, tests, and variable metadata. Exclusion semantics are unchanged ("Y"/1 = excluded, empty/NA = included).

Before	After
PKSUM1F	PKSUMXF
PKSUM1FN	PKSUMXFN
PKSUM1RS	PKSUMXRS

Additional details:

• Internal helper .derive_pksum1rs() renamed to .derive_pksumxrs() for consistency
• R/zzz.R globalVariables updated, alphabetical order preserved
• data-raw/metadata_nca_variables.csv + regenerated data/metadata_nca_variables.rda
• man/add_exclusion_reasons.Rd regenerated via devtools::document()
• New names added to inst/WORDLIST (digit-free tokens are spell-checked, unlike the old ones)

Metadata deduplication (deviation from a plain rename, flagging for review): the CSV already contained CDISC-named PKSUMXF/PKSUMXFN rows (added in #1306: "PK Summary Exclusion Flag"/"(N)", role Topic). The legacy PKSUM1F/PKSUM1FN rows were therefore removed instead of renamed, keeping the pre-existing CDISC rows. PKSUM1RS had no CDISC counterpart, so it was renamed to PKSUMXRS with label/role aligned to the same family ("PK Summary Exclusion Reason", Topic).

Definition of Done

• No PKSUM1F / PKSUM1FN / PKSUM1RS remain in the codebase
• devtools::document() run to regenerate man/ and NAMESPACE
• NEWS.md updated with the change (and the PR number)
• Package version incremented
• lintr and all unit tests pass

How to test

Rename + metadata change; all behavior covered by existing unit tests (updated expectations): test-export_cdisc.R (185 PASS), test-PKNCA.R (126 PASS), test-tlg_module.R (21 PASS). Full suite: 2005 PASS / 0 FAIL, lint 0 violations, spellcheck clean.

Contributor checklist

• Code passes lintr checks
• Code passes all unit tests
• New logic covered by unit tests
• New logic is documented
• App or package changes are reflected in NEWS
• Package version is incremented
• R script works with the new implementation (if applicable)
• Settings upload works with the new implementation (if applicable)
• If any .scss change was done, run data-raw/compile_css.R
• If a package dependency was added/changed, run data-raw/test_suggests_hidden.R

Notes to reviewer

The metadata deduplication described above is the only judgment call beyond a mechanical rename — the alternative (renaming the legacy rows) would have produced duplicate PKSUMXF/PKSUMXFN entries with conflicting labels/roles.