Make tool-managed lane splits visually transparent: collapse display + lazy-mint

Drive C_LANESCOLLAPSED=2 and hide lane buttons only on tracks the tool itself splits
(gated on the pre-write I_FREEMODE read, so user comp-lane display prefs are never
stomped). Lazy-mint managed lanes only for modes with own content, never an empty
reserved lane; confinement holds via C_LANEPLAYS on the lone lane.

src/view_mode_model.h

@@ -641,14 +641,19 @@ struct LaneMintPlan {
 //   * A track visible in exactly ONE mode (single-mode leaf, single-mode folder) stays
 //     whole-track-parked (D1) — NO split. This is the single-mode-track rule.
 //   * On a split: one TrackSplit (laneCount == number of lanes to mint), one LaneMint per
-//     involved mode, and one LaneAssign per managed-eligible OWN item onto ITS tagged
-//     mode's lane — INCLUDING pre-existing items, so a folder carrying one own Design item
-//     while derived-visible in Arrange too still lanes that item to the Design lane (it
-//     then hides+silences whenever Arrange is active). The lanes minted are the union of:
-//     the modes the track's own items belong to, PLUS the modes the track is visible in —
-//     so a folder whose own item is Design-only but which is derived-visible in Arrange
-//     mints BOTH a Design lane (holding the item) and an Arrange lane (empty, reserving
-//     the Arrange stance's slot), matching "each mode owns a fixed lane."
+//     mode the track's OWN items occupy, and one LaneAssign per managed-eligible OWN item
+//     onto ITS tagged mode's lane — INCLUDING pre-existing items, so a folder carrying one
+//     own Design item while derived-visible in Arrange too still lanes that item to the
+//     Design lane (it then hides+silences whenever Arrange is active).
+//   * LAZY-MINT: lanes are minted ONLY for modes the track's own items actually occupy —
+//     never an empty reserved lane for a mode the track is merely derived-visible in. So a
+//     folder whose own item is Design-only but which is derived-visible in Arrange mints a
+//     Design lane ONLY (holding the item), NOT an empty Arrange lane. Confinement still
+//     holds: with only a Design lane present, toggling to Arrange drives that lane's
+//     C_LANEPLAYS to 0 (hide+silence) and no lane plays, so the track reads as an empty
+//     normal track and the Design item does not leak. The Arrange lane is minted on demand
+//     when an Arrange item first lands. The derived-visibility trigger still decides WHETHER
+//     to split; it no longer inflates WHICH lanes are minted.
 //
 // Items with an empty GUID or empty modeId are skipped (defensive; a real item always
 // resolves to a mode). The function mutates nothing — it returns a plan the shell