fix: same-slot reorder is a no-op; WM_DESTROY clears card drag state; fix card_meta.h ms-rounding comment

tests/test_bank_book.cpp

@@ -973,6 +973,28 @@ static void testReorderSampleRejectsUnknown() {
     CHECK(book.pool().slots.slotOf("id-r1") == 0);       // unchanged
 }
 
+// Same-slot reorder is a true no-op: returns false, no undo point triggered,
+// JSON byte-identical before/after (the invariant that blocks spurious dirty-state).
+static void testReorderSampleSameSlotIsNoOp() {
+    BankBook book;
+    CHECK(book.pool().index.add(sampleWith("a")) == AddResult::Added);
+    CHECK(book.pool().index.add(sampleWith("b")) == AddResult::Added);
+    CHECK(book.pool().index.add(sampleWith("c")) == AddResult::Added);
+    book.reconcileSlots();  // a@0, b@1, c@2
+    const std::string jsonBefore = book.serialize();
+
+    // Drop each card onto its own current slot — must return false every time.
+    CHECK(!book.reorderSample("id-a", kPoolBankId, 0));
+    CHECK(!book.reorderSample("id-b", kPoolBankId, 1));
+    CHECK(!book.reorderSample("id-c", kPoolBankId, 2));
+
+    // Slots and JSON are byte-identical — no mutation occurred.
+    CHECK(book.pool().slots.slotOf("id-a") == 0);
+    CHECK(book.pool().slots.slotOf("id-b") == 1);
+    CHECK(book.pool().slots.slotOf("id-c") == 2);
+    CHECK(book.serialize() == jsonBefore);
+}
+
 // --- BankBook L7: Alt-replace mutator ----------------------------------------
 
 static void testReplaceSampleTakesSlotAndRemovesOccupant() {
@@ -1090,6 +1112,7 @@ int main() {
     testMigrationDefaultsToInsertionOrderDense();
     testOrderedSampleIdsReconcilesLazily();
     testReorderSampleRejectsUnknown();
+    testReorderSampleSameSlotIsNoOp();
     testReplaceSampleTakesSlotAndRemovesOccupant();
     testReplaceSampleNonDestructiveFileStays();
     testReplaceSampleRejectionsNoMutation();