Docs: reflect Phase D5 landing; close out Phase D

Archive D5 (mode switch) to COMPLETED.md and remove the completed Phase D
section from PLAN.md; add mode_switch to CLAUDE.md architecture + targets.

PLAN.md

@@ -79,35 +79,3 @@ landed milestone.
   in int64 but exceed `INT_MAX` are implementation-defined. Hardening candidate
   — add bounds check before the cast when integer-field validation is in scope.
 
----
-
-# Phase D — Design View (parallel to the M0–M11 capture roadmap)
-
-> **Separate phase namespace.** The M-numbers belong to the capture pillar
-> (M0–M11, M4 in flight). Design View is an orthogonal feature on its own track,
-> so it uses a **lettered** namespace (D1, D2, …) rather than extending the M-line.
-> This keeps the two roadmaps from colliding on numbering and reads correctly:
-> Phase D is not "the twelfth capture step," it's a different pillar. Authoritative
-> spec: **CONTEXT.md §Design View**. Product framing: `docs/product/design-view.md`.
-> When a point lands, doc-keeper moves it to `COMPLETED.md`.
-
-## D5 — in-window toggle affordance (UI)
-**Goal:** The segmented mode switch in the ReaSampler / bank_panel window header.
-CONTEXT.md §Design View (UI). **Verify (in DAW):** Segmented control shows current
-mode (lit segment), one click flips modes via the D4 toggle action, per-mode
-membership count visible, offlined-FX caveat surfaced as a tooltip.
-
-- [ ] Segmented mode switch `[ Arrange | Design ]` in the window header; active lit.
-- [ ] Wire the switch to the toggle/activate actions from D4.
-- [ ] Per-mode membership count display.
-- [ ] Offlined-FX re-init caveat as a tooltip on the switch.
-
-## Phase D open questions
-- **Snapshot durability across save-while-parked** — persisted here by decision;
-  lean alternative is force-restore-to-Arrange on save (product notes item 4).
-  (touches D3)
-- **Reconcile on delete/restructure** — tagged leaf deleted or folder restructured
-  while parked leaves a stale GUID; ignore-and-prune on next toggle/open.
-  (touches D1/D2)
-- **Screenset coexistence** — Design View drives the same flags a screenset recall
-  would; confirm last-writer-wins is not surprising. (touches D2)