visibleTracks pass 1 now evaluates every node (parent or leaf) by its own
membership, so an untagged folder carrying its own FX/media shows in Arrange
(its default) as well as any mode derived from its children. Adds case-matrix
tests; updates the nested-Arrange assertion to the corrected rule.
planToggle now enumerates all leaves in the FolderTree instead of only tagged
membership entries, so untagged (Arrange-member) tracks park in Design and
restore in Arrange. applyMode calls TrackList_AdjustWindows + UpdateArrange so
changes render immediately. Correct the stale "tagged-only" invariant comments.
Register toggle/activate/tag/untag/show-both actions driving the D2
shell and D3 model; reapply saved active mode on project load via a
persist load-signal seam. Shared guidString helper; pure nextModeId
unit-tested.
daniel