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/insert_plan.cpp

@@ -0,0 +1,49 @@
+// insert_plan.cpp — see insert_plan.h. Pure InsertMedia mode-bit arithmetic.
+
+#include "insert_plan.h"
+
+namespace reasampler {
+
+namespace {
+
+// Base target bits (mode&3). We use only 0 (current track) and 1 (new track).
+constexpr int kBaseCurrentTrack = 0;  // add to current track
+constexpr int kBaseNewTrack     = 1;  // add new track
+
+// Tempo-conform bits, verbatim from the header doc-comment.
+constexpr int kMatchTempo1x   = 8;    // &8:  try to match tempo 1x
+constexpr int kMatchTempoHalf = 16;   // &16: try to match tempo 0.5x
+constexpr int kMatchTempoDbl  = 32;   // &32: try to match tempo 2x
+constexpr int kDontPreservePitch = 64; // &64: don't preserve pitch when matching tempo
+
+}  // namespace
+
+int computeInsertMode(const InsertOptions& opts) {
+    int mode = opts.target == InsertTarget::NewTrack ? kBaseNewTrack
+                                                     : kBaseCurrentTrack;
+
+    switch (opts.conform) {
+        case TempoConform::None:
+            // No tempo bits: native length, no stretch. (Also never &4.)
+            return mode;
+        case TempoConform::Ratio1x:
+            mode |= kMatchTempo1x;
+            break;
+        case TempoConform::RatioHalf:
+            mode |= kMatchTempoHalf;
+            break;
+        case TempoConform::RatioDouble:
+            mode |= kMatchTempoDbl;
+            break;
+    }
+
+    // Tempo bits are set (conform != None). Add the pitch-shift bit only when the
+    // caller asked NOT to preserve pitch. When conform == None we already returned
+    // above, so this can never fire without a tempo bit present.
+    if (!opts.preservePitch)
+        mode |= kDontPreservePitch;
+
+    return mode;
+}
+
+} // namespace reasampler