Docs: reflect Phase D4 landing

Archive D4 (actions + integration) to COMPLETED.md; add track_guid and
Design View action family to CLAUDE.md architecture.

PLAN.md

@@ -127,17 +127,6 @@ landed milestone.
 > spec: **CONTEXT.md §Design View**. Product framing: `docs/product/design-view.md`.
 > When a point lands, doc-keeper moves it to `COMPLETED.md`.
 
-## D4 — actions
-**Goal:** Bindable action set for the mode workflow. CONTEXT.md §Design View
-(actions). **Verify (in DAW):** Each action registered (bindable in Actions list);
-toggle + mode-jumps MIDI-bindable; tag/untag acts on the current track selection.
-
-- [ ] Toggle active mode (cycle; extensible to cycle-all for >2 modes).
-- [ ] Activate mode: Arrange / Activate mode: Design (direct jumps).
-- [ ] Tag selected tracks → Design / → Arrange; Untag selected (= → Arrange).
-- [ ] Show-both for selected tracks (toggle).
-- [ ] Register each (`command_id`/`gaccel`/`hookcommand`); toggle + jumps MIDI-bindable.
-
 ## 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