fix: insert onto selected track(s) at edit cursor, not a new track

Wire both insert actions to CurrentTrack (InsertMedia base 0). Per selected
track: SetOnlyTrackSelected, reset cursor to snapshot, InsertMedia; then restore
original selection + cursor. No-op when no track selected. Corrected stale
extraflags / new-track / preservePitch comments.

src/insert_plan.h

@@ -46,15 +46,15 @@ enum class TempoConform {
     RatioDouble,// &32: try to match tempo 2x
 };
 
-// Options that shape one InsertMedia call. Defaults encode the safe path:
-// new track, no conform, pitch preserved.
+// Options that shape one InsertMedia call. Defaults encode the intended path:
+// current track (user's selection), no conform, pitch preserved.
 struct InsertOptions {
-    InsertTarget target = InsertTarget::NewTrack;
+    InsertTarget target = InsertTarget::CurrentTrack;
     TempoConform conform = TempoConform::None;
 
-    // Only meaningful when conform != None. When true, adds &64 ("don't preserve
+    // Only meaningful when conform != None. When false, adds &64 ("don't preserve
     // pitch when matching tempo") so a tempo match also shifts pitch (classic
-    // varispeed). Default false = preserve pitch across the tempo match. Ignored
+    // varispeed). Default true = preserve pitch across the tempo match. Ignored
     // when conform == None (no tempo bits set, so pitch is moot).
     bool preservePitch = true;
 };