Docs: reflect Phase D2/D3 landing

Archive D2 (view shell) and D3 (persist slice) to COMPLETED.md;
add view/view_tree to CLAUDE.md architecture + view_tree_tests target.

PLAN.md

@@ -127,36 +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`.
 
-## D2 — view shell (apply flags in the DAW)
-**Goal:** Read the folder tree and drive REAPER flags per the planner.
-CONTEXT.md §Design View (view shell, REAPER API surface).
-**Verify (in DAW):** Toggling active mode hides + parks inactive leaves
-(`B_SHOWINTCP`/`B_SHOWINMIXER`/`B_MAINSEND`/`I_FXEN` + per-FX offline) and restores
-active ones from snapshot. **Master untouched. `B_MUTE`/`I_SOLO` untouched.**
-Untagged tracks untouched. Parents follow their tagged descendants.
-
-- [ ] Build parent↔child tree from `I_FOLDERDEPTH`; feed to `view_mode_model`.
-- [ ] Snapshot prior flag values (`GetMediaTrackInfo_Value`) before parking.
-- [ ] Apply park/restore ops (`SetMediaTrackInfo_Value` for the four flags;
-  `TrackFX_GetCount` + per-FX `TrackFX_SetOffline`). Verify flag names/signatures.
-- [ ] GUID resolution: `GetTrackGUID` / `guidToString` / `stringToGuid` (never index).
-- [ ] Review gate: no path touches master visibility or `B_MUTE`/`I_SOLO`, or any
-  untagged track's owned flags.
-
-## D3 — persist slice (view state ↔ project ext state)
-**Goal:** Serialize the view section into the `"reasampler"` namespace alongside the
-bank; reapply the active mode on project open. CONTEXT.md §Design View (persist).
-**Verify (in DAW):** Membership + active mode + snapshots survive Save / Save As /
-close+reopen; on open, the active mode's visibility + processing is reapplied.
-Saved-while-parked project restores parked tracks from persisted snapshots (not to a
-guessed "on").
-
-- [ ] Serialize/deserialize the view section under `"reasampler"` (shared blob,
-  distinct section from the bank index).
-- [ ] Reapply active mode on project open (rebuild tree, run the planner).
-- [ ] Confirm survival across Save / Save As; snapshot durability across
-  save-while-parked.
-
 ## 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);