fix: use true Jaccard similarity in wordOverlap (intersection/union)

Replaces max(|A|,|B|) denominator with |A∪B| = |A|+|B|-intersection,
which is the correct Jaccard formula and avoids inflating similarity
when both name sets have significant unique words.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/import-hk-parishes.ts

@@ -396,9 +396,10 @@ function wordOverlap(a: string, b: string): number {
   const setA = new Set(a.split(' ').filter(Boolean));
   const setB = new Set(b.split(' ').filter(Boolean));
   if (setA.size === 0 || setB.size === 0) return 0;
-  let common = 0;
-  for (const w of setA) if (setB.has(w)) common++;
-  return common / Math.max(setA.size, setB.size);
+  let intersection = 0;
+  for (const w of setA) if (setB.has(w)) intersection++;
+  const union = setA.size + setB.size - intersection;
+  return intersection / union;
 }
 
 /**