fix(card_drag): extend drop rects one trailing row so beyond-extent drops land

computeSlotRectsForDrop adds cols slots past maxSlot; classifyCardDrag and
drawCardDropTarget use it instead of the render rects. doReorderDrop already
accepts any non-negative slot. New tests cover beyond-extent resolves,
empty-bank trailing row, gap pixels still miss, empty-cell within-extent works.

tests/test_bank_book.cpp

@@ -995,6 +995,26 @@ static void testReorderSampleSameSlotIsNoOp() {
     CHECK(book.serialize() == jsonBefore);
 }
 
+// Reorder to a slot BEYOND the current maxSlot — the trailing-drop case. The model
+// places the card at the exact target slot (no shift, slot is empty), leaving the card's
+// old slot empty. This is the pure-layer gate for the DAW beyond-extent drop repro.
+static void testReorderSampleBeyondMaxSlot() {
+    BankBook book;
+    CHECK(book.pool().index.add(sampleWith("a")) == AddResult::Added);
+    CHECK(book.pool().index.add(sampleWith("b")) == AddResult::Added);
+    book.reconcileSlots();  // a@0, b@1 -> maxSlot = 1
+    CHECK(book.pool().slots.maxSlot() == 1);
+
+    // Drag "b" to slot 5 (well beyond maxSlot=1). Should succeed: slot 5 is empty,
+    // no insert-shift needed, b just takes slot 5.
+    CHECK(book.reorderSample("id-b", kPoolBankId, 5));
+    CHECK(book.pool().slots.slotOf("id-b") == 5);
+    CHECK(book.pool().slots.slotOf("id-a") == 0);  // a untouched
+    // maxSlot is now 5; slot 1 is empty (gap).
+    CHECK(book.pool().slots.maxSlot() == 5);
+    CHECK(book.pool().slots.idAt(1).empty());  // b's old slot is now a gap
+}
+
 // --- BankBook L7: Alt-replace mutator ----------------------------------------
 
 static void testReplaceSampleTakesSlotAndRemovesOccupant() {
@@ -1113,6 +1133,7 @@ int main() {
     testOrderedSampleIdsReconcilesLazily();
     testReorderSampleRejectsUnknown();
     testReorderSampleSameSlotIsNoOp();
+    testReorderSampleBeyondMaxSlot();
     testReplaceSampleTakesSlotAndRemovesOccupant();
     testReplaceSampleNonDestructiveFileStays();
     testReplaceSampleRejectionsNoMutation();