eucast_rules(): add add_if_missing argument to control NA imputation (#259)

When `add_if_missing = FALSE`, rules are only applied to cells that already
contain an SIR value; `NA` cells are left untouched. This is useful with
`overwrite = TRUE` to update reported results without imputing values for
drugs that were not tested.

https://claude.ai/code/session_01Nucc8nXGLqNUjtuC9GrhTc

NEWS.md

@@ -1,3 +1,4 @@
+<<<<<<< HEAD
 # AMR 3.0.1.9044
 
 ### New
@@ -16,7 +17,9 @@
   - Functions such as `susceptibility()` count WT as S and NWT as R
 * Function `interpretive_rules()`, which allows future implementation of CLSI interpretive rules (#235)
   - `eucast_rules()` has become a wrapper around that function
+  - Gained argument `add_if_missing` (default: `TRUE`). When set to `FALSE`, rules are only applied to cells that already contain an SIR value; `NA` cells are left untouched. This is useful with `overwrite = TRUE` to update reported results without imputing values for drugs that were not tested (#259)
 * Function `amr_course()`, which allows for automated download and unpacking of a GitHub repository for e.g. webinar use
+* Two new `NA` objects, `NA_ab_` and `NA_mo_`, analogous to base R's `NA_character_` and `NA_integer_`, for use in pipelines that require typed missing values
 
 ### Fixes
 * Fixed a bug in `as.sir()` where values that were purely numeric (e.g., `"1"`) and matched the broad SIR-matching regex would be incorrectly stripped of all content by the Unicode letter filter