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.

COMPLETED.md

@@ -141,3 +141,37 @@ mode.
 - [x] Tests: N-mode add/query; parent follows tagged leaf (multi-mode parent);
   restore-round-trip returns snapshot values (never hardcoded "on"); show-both leaf
   never parked; unknown/stale GUID tolerated; JSON lossless.
+
+---
+
+## 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.
+
+- [x] Build parent↔child tree from `I_FOLDERDEPTH`; feed to `view_mode_model`.
+- [x] Snapshot prior flag values (`GetMediaTrackInfo_Value`) before parking.
+- [x] Apply park/restore ops (`SetMediaTrackInfo_Value` for the four flags;
+  `TrackFX_GetCount` + per-FX `TrackFX_SetOffline`). Verify flag names/signatures.
+- [x] GUID resolution: `GetTrackGUID` / `guidToString` / `stringToGuid` (never index).
+- [x] 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").
+
+- [x] Serialize/deserialize the view section under `"reasampler"` (shared blob,
+  distinct section from the bank index).
+- [x] Reapply active mode on project open (rebuild tree, run the planner).
+- [x] Confirm survival across Save / Save As; snapshot durability across
+  save-while-parked.