PK helpers: remove dead postdose arg, fix geom_mean, vectorize formatters, add tests

[Melkiades]

Summary

The PK helper functions added in #289 (R/pk_helper.R) have a few defects and gaps that should be addressed.

1. imputation_rules() — postdose argument is dead code

The two 1/3 rule branches (!postdose and postdose) are byte-for-byte identical, so the postdose flag has no effect on the output. The argument adds a required parameter and implies behaviour that does not exist.

Post-dose vs pre-dose handling is a data step (e.g. substituting post-dose LTRs with half the LLOQ before summarizing), not a display decision. That logic belongs upstream in the caller/template, before statistics are computed. imputation_rules() should only decide how an already-computed statistic is displayed given the BLQ ratio.

Proposed fix: remove postdose from the signature and collapse the two identical branches into one.

2. geom_mean() returns NaN for all-NA input

geom_mean(c(NA, NA)) returns NaN rather than NA_real_. The any(x <= 0, na.rm = na.rm) guard can also propagate NA into if() when na.rm = FALSE.

Proposed fix: drop NAs up front (when na.rm = TRUE), return NA_real_ for the empty/all-non-positive cases.

3. Formatters are not vectorized and bake in domain logic

fmt_3sig() and fmt_pct() accept only length-1 input and return the string "NE" for NA. "Not estimable" is analysis semantics, not formatting — mixing it into a generic formatter is surprising and makes the functions hard to reuse.

Proposed fix: vectorize both formatters and return NA_character_ for NA/non-finite input. The "NE"/"ND" substitution stays in imputation_rules() where it belongs.

4. Missing documentation

cv(), geom_cv(), and fmt_pct() lack @description and lifecycle badges that the other helpers have.

5. No test coverage

R/pk_helper.R shipped without a tests/testthat/test-pk_helper.R.

Scope

All of the above will be fixed in one PR. The public API change is the removal of postdose from imputation_rules(); the dependent tlg.templates PKCT01 template call will be updated separately.