docs(design-view): parent visibility = descendant-derived OR own membership

Untagged folder carrying its own FX/media shows in Arrange (its default) as
well as any mode derived from its children. Parents still never parked.

CONTEXT.md

@@ -197,9 +197,15 @@ enforces the separation **without ever destroying the user's real state.**
 - **Membership.** Default = Arrange; every untagged leaf belongs to it. Leaves
   **opt in** to Design (or any mode). No track appears in two modes at once except
   (a) via an explicit **show-both** toggle, or (b) parent/folder derivation.
-- **Parents are derived, never tagged.** A parent appears in — and is never parked
-  in — every mode any of its descendant leaves belongs to. Rule of thumb: **tag
-  leaves; parents follow.** Master track is always visible and never touched.
+- **Parents are derived, never tagged.** A parent/folder track is visible in mode M
+  if **either** (a) any descendant leaf is visible in M (descendant-derived), **or**
+  (b) the parent belongs to M by its **own membership** — and an untagged parent is
+  an Arrange member by default. So an untagged folder carrying its own FX/media
+  above all-Design leaves shows in **both** Arrange (its own default) and Design
+  (derived from its children). A parent is **never parked** in any mode it is visible
+  in. Rule of thumb: **tag leaves; parents follow** — the common case, since a
+  content-bearing folder still surfaces wherever its own membership places it.
+  Master track is always visible and never touched.
 - **N-mode model, two-mode UI.** The data model carries arbitrarily many modes; the
   UI ships **Arrange** + **Design**. A mode is (stable id, display name, ordinal).
   Arrange is special only as the default home for untagged leaves.