Band Lineup 1.12.0: Sync keeps your moves and puts each person in one spot

Two Sync problems found by checking the real October 25 plan against the
board:

- Someone on the plan twice (Mike Powell is Leader and Guitar [Acoustic])
  could fill two spots, pushing a real player off the board. Which spot
  the duplicate took depended on the order Planning Center returned
  people, so it only happened some weeks. Assignment now uses each person
  once.
- Moving a synced person to another spot was undone by the next Sync,
  which re-placed everyone it had filled. A move is now a hand placement
  for the current plan and Sync leaves it alone.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

modules/band-lineup/matching.js

@@ -128,7 +128,11 @@ function assignMembersToSlots(slots, members) {
       if (score > bestScore) { bestScore = score; bestIndex = i; }
     }
     if (bestIndex < 0) return null;
-    return pool.splice(bestIndex, 1)[0];
+    const chosen = pool.splice(bestIndex, 1)[0];
+    // One spot per person: someone scheduled twice (Leader + Acoustic) must not fill a second spot.
+    const key = chosen.personId || '';
+    if (key) for (let i = pool.length - 1; i >= 0; i -= 1) if (pool[i].personId === key) pool.splice(i, 1);
+    return chosen;
   });
 }