docs(mimic-iv-ed): add pyxis-before-intime audit query

Give researchers a concrete count for the #1910 offset pattern.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: Chessing234

mimic-iv-ed/concepts/pyxis_before_intime.sql

@@ -0,0 +1,14 @@
+-- Count Pyxis rows whose charttime falls before the linked ED stay intime.
+-- Useful as a sensitivity check when building ED medication cohorts (#1910).
+-- Dialect: PostgreSQL / BigQuery-compatible with minor cast tweaks.
+
+SELECT
+  COUNT(*) AS pyxis_rows,
+  COUNT(*) FILTER (WHERE p.charttime < e.intime) AS before_intime,
+  COUNT(*) FILTER (
+    WHERE p.charttime < e.intime
+      AND p.charttime >= e.intime - INTERVAL '2 hours'
+  ) AS before_intime_within_2h
+FROM ed.pyxis AS p
+INNER JOIN ed.edstays AS e
+  ON p.stay_id = e.stay_id;