feat(m11): native OS drag-out of bank samples (copy-only)

Pure drag_out (gesture boundary + path-list assembly) + Windows OLE
CF_HDROP / SWELL file-list shell; bank_panel hands off when a dragged
selection leaves the client area. COPY-ONLY mask; internal drag intact.

src/bank_panel.cpp

@@ -48,6 +48,8 @@
 
 #include "action_buttons.h"  // pure button-strip layout + label format (M11)
 #include "actions.h"    // persistBankOp — shared undo-block wrapper (R-B panel path)
+#include "drag_out.h"        // pure gesture-boundary decision + path-list assembly (M11)
+#include "drag_out_win.h"    // OLE / SWELL drag-out initiation seam (M11)
 #include "app_version.h"     // channelCommandId — compose the named-command lookup string (M11)
 #include "bank_book.h"
 #include "bank_grid.h"
@@ -1616,6 +1618,34 @@ std::vector<std::string> focusedSelectionIds() {
     return ids;
 }
 
+// Resolves the ARMED drag payload (g_panel.dragSampleIds, from g_panel.dragSourceBankId) to
+// the absolute, existing-file path list for a native OS drag-out (M11). Reuses the SAME M4
+// path machinery the panel uses for audition/insert (resolveBankFile over the current
+// project dir) — no temp copies; the drag points straight at the on-disk bank files. Each
+// id is looked up in its SOURCE bank's index (the payload's origin, not the focused region,
+// which can differ once the pointer roams), resolved, stat'd, then handed to the pure
+// drag_out::assemblePathList for dedupe + skip-missing/unresolved policy. Read-only: no
+// mutation of sample / index / selection (invariant #2).
+std::vector<std::string> resolveDragPathsForOs() {
+    std::vector<ResolvedSample> resolved;
+    BankBook* b = book();
+    if (!b) return {};
+    const BankIndex* idx = b->index(g_panel.dragSourceBankId);
+    if (!idx) return {};
+
+    const std::string projectDir = currentProjectDir();
+    resolved.reserve(g_panel.dragSampleIds.size());
+    for (const std::string& sid : g_panel.dragSampleIds) {
+        const Sample* s = idx->query(sid);
+        if (!s) continue;  // stale id — the pure layer would skip it anyway; nothing to resolve
+        ResolvedSample rs;
+        rs.absolutePath = resolveBankFile(projectDir, s->relativePath);
+        rs.fileExists = !rs.absolutePath.empty() && fs::exists(fs::path(rs.absolutePath));
+        resolved.push_back(std::move(rs));
+    }
+    return assemblePathList(resolved).paths;
+}
+
 // --- Popup menus --------------------------------------------------------------
 //
 // SWELL/Win32 both expose CreatePopupMenu / InsertMenu (SWELL aliases SWELL_InsertMenu
@@ -2076,6 +2106,37 @@ void onMouseMove(int x, int y) {
         }
     }
     if (g_panel.dragging) {
+        // M11 gesture boundary (invariant #4): while a drag with samples is under way, the
+        // moment the pointer LEAVES the panel client area the gesture becomes OS-bound —
+        // hand the payload to the native OS drag. Inside the client area it stays the
+        // existing internal bank-to-bank drag, byte-identical. The boundary decision is the
+        // pure drag_out::decideGesture (drag state + pointer + client rect).
+        RECT cr{};
+        GetClientRect(g_panel.hwnd, &cr);
+        const PanelClientRect client{cr.left, cr.top, cr.right - cr.left, cr.bottom - cr.top};
+        const DragState st{/*dragging=*/true, /*hasArmedSamples=*/!g_panel.dragSampleIds.empty()};
+        if (decideGesture(x, y, client, st) == DragGesture::OsDrag) {
+            // Resolve the payload to existing on-disk paths BEFORE tearing down internal
+            // drag state (the resolver reads dragSourceBankId / dragSampleIds).
+            const std::vector<std::string> paths = resolveDragPathsForOs();
+
+            // Reset internal drag state and release capture NOW: DoDragDrop runs its own
+            // modal loop and takes over mouse capture, so the internal drag must be fully
+            // wound down first (no stale dragging/dropKind, no lingering SetCapture). A
+            // cancelled/empty OS drag therefore leaves the panel in a clean, no-op state
+            // (invariant #2 — nothing mutated).
+            if (GetCapture() == g_panel.hwnd) ReleaseCapture();
+            g_panel.dragArmed = false;
+            g_panel.dragging = false;
+            g_panel.dropKind = DropKind::None;
+            g_panel.dropBankId.clear();
+            invalidatePanel();
+
+            // Empty path list -> nothing draggable (all stale/missing); do not start a drag.
+            if (!paths.empty())
+                initiateDragOut(g_panel.hwnd, paths);  // COPY-ONLY; blocking on Windows
+            return;
+        }
         updateDropTarget(x, y);
         invalidatePanel();
     }