feat: add M6 insert — place selected bank sample at edit cursor

InsertMedia-driven placement reading the bank panel selection, undo-wrapped.
Native length by default; conform-to-tempo is an explicit second action, never
a silent stretch. Pure mode-bit logic (insert_plan) unit-tested.

src/bank_panel.cpp

@@ -778,6 +778,22 @@ bool bankPanelIsOpen() {
     return g_panel.open;
 }
 
+std::vector<std::string> bankPanelSelectedSampleIds() {
+    std::vector<std::string> ids;
+    const BankIndex* bank = g_panel.session ? &g_panel.session->bank() : nullptr;
+    if (!bank) return ids;
+    const std::vector<Sample>& samples = bank->all();
+    const int count = static_cast<int>(samples.size());
+    // selection.indices is sorted-ascending unique (bank_grid invariant), so the
+    // returned ids come out in bank order. Guard each index against the live count
+    // in case the selection outran a shrink the fingerprint pass hasn't cleared yet.
+    for (int idx : g_panel.selection.indices) {
+        if (idx >= 0 && idx < count)
+            ids.push_back(samples[static_cast<std::size_t>(idx)].id);
+    }
+    return ids;
+}
+
 void bankPanelRefresh() {
     if (!g_panel.open || !g_panel.hwnd) return;
     // Repaint only when the bank actually changed (generation bump). Cheap tick