docs: record D2-W3-B landing; mark Phase D2 complete; preserve deferred panel indicator

Move D2-W3-B (item-level mode actions + W3-A polish) from PLAN.md to COMPLETED.md.
Phase D2 two-canvas sub-phase functionally complete; panel UI indicator explicitly
deferred, not dropped.

COMPLETED.md

@@ -614,3 +614,43 @@ flips to fixed-lane mode, and whether the leftover empty default lane 0 is silen
   closed with no label (discarded by REAPER) when the plan is non-empty but every write
   was already satisfied, so no phantom undo points accumulate.
 - **Review passed** with no Critical or Major findings.
+
+---
+
+## D2-W3-B — item-level mode actions + W3-A polish
+**Goal:** Item-level lane/mode-management actions mirroring the track-level Design
+View tag family (bindable in the Actions list), plus the three code-review polish
+items carried from D2-W3-A. The persist slice and lane-ownership index round-trip
+were completed in D2-W3-A; this wave closes the remaining action surface and
+cleans up the implementation.
+See CONTEXT.md §Two-canvas sub-phase (Module architecture — persistence).
+**Verify (in DAW):** Item mode actions registered and MIDI-bindable in the Actions
+list; re-drive mint/apply so each item lands on its mode's managed lane; manual-lane
+items exempt; one undo block per action. ctest 14/14 green.
+**Depends on:** D2-W3-A.
+
+- [x] "Move selected items → Design" action (`CEREBELLUM_REASAMPLER_VIEW_` family):
+  retags selected items' membership to Design mode, re-drives the existing mint/apply
+  so each item lands on its mode's managed lane; manual-lane items exempt; one undo
+  block.
+- [x] "Move selected items → Arrange" action: retags selected items' membership to
+  Arrange mode, re-drives mint/apply; manual-lane items exempt; one undo block.
+- [x] "Untag selected items" action: removes selected items' membership, re-drives
+  mint/apply; manual-lane items exempt; one undo block.
+- [x] All three registered (`command_id`/`gaccel`/`hookcommand`); MIDI-bindable.
+- [x] W3-A polish — simplified `applyMintPlan`'s redundant `I_NUMFIXEDLANES` re-read:
+  single grow-and-track pass removes the second `GetMediaTrackInfo_Value` call inside
+  the mint loop.
+- [x] W3-A polish — extracted shared item-read seam (`src/item_read.{h,cpp}`):
+  removes duplicated `itemGuid`/`itemLaneName` read logic from `view.cpp` and
+  `bank_panel.cpp`.
+- [x] W3-A polish — added reconcile guard in `reconcileManagedLanes`: skips lanes
+  encoding an unregistered mode id (log and skip rather than silently recording an
+  orphaned ownership entry).
+
+**Notes/decisions:**
+- ctest 14/14 green; review passed with no Critical or Major findings.
+- **Panel UI indicator explicitly deferred** (Daniel's decision): a per-track
+  lane-split marker has no natural cheap home in the bank panel; the mode switch
+  already shows the active mode. Preserved as a deferred/backlog note in PLAN.md
+  Phase D2 — not silently dropped.