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.

COMPLETED.md

@@ -239,6 +239,28 @@ toggle + mode-jumps MIDI-bindable; tag/untag acts on the current track selection
 
 ---
 
+## 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.
+
+- [x] Segmented mode switch `[ Arrange | Design ]` in the window header; active lit.
+- [x] Wire the switch to the toggle/activate actions from D4.
+- [x] Per-mode membership count display.
+- [x] Offlined-FX re-init caveat as a tooltip on the switch.
+
+**Notes/decisions:**
+- New PURE module `src/mode_switch.{h,cpp}` — REAPER-free layout math for the
+  Design View mode switch: divides a header rectangle into N equal segments (one per
+  registered mode) and hit-tests a point to a segment. Unit-tested via a new
+  `mode_switch_tests` CTest target. Mirror of `bank_grid`.
+- `src/bank_panel.cpp` — segmented `[ Arrange | Design ]` control drawn in the panel
+  header (one lit segment per registered mode, click activates that mode via
+  `view::applyMode`), with the grid offset below the header.
+
+---
+
 ## Milestone 7 — capture action family
 **Goal:** Bindable capture actions for master / selected tracks / selected items /
 razor area, each with wet-dry + tail options. CONTEXT.md §actions, Build order 7.