feat(normalizer): year expansion century rule

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tools/import-normalizer/dates.py

@@ -39,6 +39,25 @@ def resolve_feast_or_season(token: str, year: int):
     return None
 
 
+def expand_year(token: str):
+    """Expand a 2/3/4-digit year string per the 1873–1957 century rule. None if ambiguous."""
+    token = token.strip()
+    if not token.isdigit():
+        return None
+    n, v = len(token), int(token)
+    if n == 4:
+        return v
+    if n == 3:
+        return 1000 + v
+    if n == 2:
+        if v <= config.TWO_DIGIT_19XX_MAX:
+            return 1900 + v
+        if v >= config.TWO_DIGIT_18XX_MIN:
+            return 1800 + v
+        return None
+    return None
+
+
 def easter(year: int) -> datetime.date:
     """Easter Sunday (Gregorian) via the Anonymous Gregorian / Butcher algorithm."""
     a = year % 19