Fix as.sir() data.frame: preserve already-<sir> columns, exclude metadata

Issue #278: two related bugs in the column-detection / type-assignment pipeline.

Bug 1 – already-<sir> columns deleted on re-run
  Line 886 excluded already-sir columns from the type assignment (they
  stayed type "") causing the result loop to do x[,col] <- NULL, deleting
  them.  Fix: drop the !is.sir() guard so all untyped columns fall through
  to type "sir" and are re-processed correctly.

Bug 2 – metadata columns treated as antibiotics
  as.ab("patient") -> OXY, as.ab("ward") -> PRU.  The column detector
  accepted any column whose name matched an antibiotic code, regardless of
  content.  Fix: for name-matched columns that do not already carry an AMR
  class, also verify content looks like AMR data (all_valid_mics, all-
  numeric, or any SIR-like string).  all_valid_disks() is intentionally
  avoided here because it strips letters from strings (as.disk("Pt_1")==1).

Also adds tools/benchmark_parallel.R: a standalone script that times
sequential vs parallel as.sir() across n=20/200/2000/20000 rows and
saves a ggplot2 PNG to tools/benchmark_parallel.png.

https://claude.ai/code/session_012DXCXbZUC54Zij1z9bFiHR

NEWS.md

@@ -35,6 +35,8 @@
 * Fixed SIR and MIC coercion of combined values, e.g. `as.sir("<= 0.002; S") ` or `as.mic("S; 0.002")` (#252)
 * Fixed translation of foreign languages in `sir_df()` (#272)
 * Fixed BRMO classification by including bacterial complexes (#275)
+* Fixed `as.sir()` for data frames silently deleting columns whose AB class was already `<sir>` when called a second time (re-running on already-converted data) (#278)
+* Fixed `as.sir()` for data frames incorrectly treating metadata columns (e.g. `patient`, `ward`) as antibiotic columns when their names coincidentally matched an antibiotic code; column content is now validated against AMR data patterns before inclusion
 
 ### Updates
 * Extensive `cli` integration for better message handling and clickable links in messages and warnings (#191, #265)