fix(view): lane-separate a content-bearing folder's own media when it's derived-visible in >1 mode

planLaneMinting now takes the model + folder tree and splits a track visible in
2+ modes that carries own media (own-item span OR folder derived-visibility),
fixing an item dropped on a cross-mode folder leaking into every mode. Show-both
tracks are never force-split.

src/view.h

@@ -52,13 +52,16 @@ namespace reasampler {
 // registered mode. `proj` may be nullptr to mean REAPER's current project.
 bool applyMode(ViewModeModel& model, const std::string& targetModeId, ReaProject* proj);
 
-// Mints managed fixed lanes for any track in `proj` that now holds content of MORE
-// THAN ONE mode, and assigns each item to its mode's managed lane (Phase D2 Wave 3).
+// Mints managed fixed lanes for any track in `proj` that is VISIBLE IN MORE THAN ONE
+// MODE while carrying its own media, and assigns each item to its mode's managed lane
+// (Phase D2 Wave 3; visibility trigger added by the folder-media fix).
 //   1. Enumerates every track + its items; resolves each item's mode from the model's
 //      membership (untagged ⇒ Arrange) and reads whether it currently sits on a MANUAL
-//      lane (exempt).
-//   2. Runs the pure planLaneMinting decision. A track with content of only one mode
-//      is left whole-track-parked (D1) — NOT lane-split.
+//      lane (exempt). Builds the FolderTree (I_FOLDERDEPTH) so derived visibility counts.
+//   2. Runs the pure planLaneMinting decision (model + tree aware). A track visible in
+//      exactly one mode is left whole-track-parked (D1) — NOT lane-split. A track visible
+//      in >1 mode while carrying own media splits: its own items span modes, OR it is a
+//      content-bearing folder derived-visible across modes. show-both tracks never split.
 //   3. For each track that must split: enables fixed-lane mode (I_FREEMODE=2), ensures
 //      enough fixed lanes (I_NUMFIXEDLANES), stamps each managed lane's durable name
 //      (P_LANENAME:n), records the lane MANAGED-for-its-mode in the model's ownership