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

src/bank_book.cpp

@@ -76,6 +76,7 @@ bool SlotMap::remove(const std::string& id) {
 bool SlotMap::reorder(const std::string& id, int targetSlot) {
     if (slotOf(id) < 0) return false;  // not mapped -> no mutation
     if (targetSlot < 0) targetSlot = 0;
+    if (slotOf(id) == targetSlot) return false;  // already there — true no-op
 
     // Detach the moving id first so the occupancy test below sees the post-move world.
     remove(id);

src/bank_panel.cpp

@@ -2922,6 +2922,7 @@ void onMouseMove(int x, int y) {
             g_panel.dropBankId.clear();
             g_panel.cardGesture = CardGesture::None;
             g_panel.dragTargetSlot = -1;
+            g_panel.dragPrimaryId.clear();
             invalidatePanel();
 
             // Empty path list -> nothing draggable (all stale/missing); do not start a drag.
@@ -3117,6 +3118,9 @@ WDL_DLGRET dlgProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam) {
             stopAudition();
             g_panel.selection = Selection{};
             g_panel.dragArmed = g_panel.dragging = false;
+            g_panel.cardGesture = CardGesture::None;
+            g_panel.dragTargetSlot = -1;
+            g_panel.dragPrimaryId.clear();
             g_panel.hovered = Hover{};
             g_panel.tooltipShown = false;
             g_panel.hwnd = nullptr;

src/card_meta.h

@@ -47,7 +47,7 @@ struct MusicalLength {
 std::string formatBarsBeats(const MusicalLength& m);
 
 // seconds.milliseconds from a wall-clock length (always derivable, meter-independent).
-//   * "S.mmm" — integer seconds, a dot, zero-padded 3-digit milliseconds (floored).
+//   * "S.mmm" — integer seconds, a dot, zero-padded 3-digit milliseconds (rounded to nearest ms).
 //     e.g. 0.0 -> "0.000", 1.5 -> "1.500", 62.037 -> "62.037".
 //   * negative length is clamped to "0.000" (a length is never negative; defensive).
 std::string formatSecondsMs(double lengthSeconds);

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();