docs(plan): record Phase D2 two-canvas waves; archive landed D2-W1

Add a Phase D2 (two-canvas) section to PLAN.md with the open D2-W2
(shell) and D2-W3 (actions, persist, UI) waves in house style. Archive
the landed D2-W1 pure view_mode_model lane extension in COMPLETED.md.

COMPLETED.md

@@ -261,6 +261,29 @@ membership count visible, offlined-FX caveat surfaced as a tooltip.
 
 ---
 
+## D2-W1 — view_mode_model lane extension (pure)
+**Goal:** Extend D1's pure planner to item level for the two-canvas sub-phase:
+lane↔mode mapping, a managed-vs-manual lane-ownership index, managed-only item-lane
+ops in the toggle planner, the "which lanes may this toggle touch" query, the
+auto-tag decision (manual-lane items exempt; pre-existing ⇒ Arrange), and JSON
+round-trip of the lane index. REAPER-free, unit-tested; mirror of D1. CONTEXT.md
+§Two-canvas sub-phase (Module architecture — pure).
+**Verify:** CTest green; D1 behavior and tests unchanged.
+
+- [x] Lane↔mode mapping: which lane maps to which mode, which `C_LANEPLAYS` value
+  per mode.
+- [x] Lane-ownership index: per (track GUID, lane) managed-which-mode vs manual;
+  managed-only item-lane op family alongside the existing track-flag op family.
+- [x] "Which lanes may this toggle touch" query (managed only) — planner emits lane
+  ops for managed lanes only, never for manual lanes.
+- [x] Auto-tag decision (pure): new track/item GUIDs + active mode ⇒ membership
+  writes; manual-lane items exempt; pre-existing ⇒ Arrange.
+- [x] JSON round-trip of the lane-ownership index.
+- [x] Tests: managed/manual partition; toggle-touches-managed-only; auto-tag
+  exemption for manual-lane items; JSON lossless; D1 behavior/tests unchanged.
+
+---
+
 ## 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.

PLAN.md

@@ -151,6 +151,67 @@ then trims at the -72 dB decay point (± inherent realtime tolerance); realtime
 
 ---
 
+# Phase D2 — Two-canvas (item-level mode projection; additive to D1)
+
+> **Design View sub-phase.** Extends D1's track-level mode projection to **item
+> level** via REAPER 7 fixed lanes: on a track present in both stances, each mode
+> owns a fixed lane — the active mode's lane shows and plays, the inactive mode's is
+> hidden and silenced — so a Design take and an Arrange take can share the same
+> track and time position without colliding on the view. Nothing in D1 changes.
+> Runtime floor rises to **REAPER 7** for this sub-phase (no version-gate branch;
+> below v7 it is simply unavailable). Authoritative spec: **CONTEXT.md §Two-canvas
+> sub-phase (Phase D2 / Phase E)** and the surrounding §Design View — additive phase
+> spec. Product framing: `docs/product/design-view.md` §Two-canvas direction. When a
+> point lands, doc-keeper moves it to `COMPLETED.md`.
+>
+> **D2-W1 (pure lane extension) has landed** — see `COMPLETED.md`.
+
+## D2-W2 — shell: lane application + new-content detection
+**Goal:** The view shell applies the planner's managed-lane ops in the DAW and the
+bank_panel timer detects new content and auto-tags it to the active mode. Resolves
+the two flagged implementation design points (I_FIXEDLANE reorder/renumber
+fragility; the auto-tag / manual-lane detection heuristic). See CONTEXT.md
+§Two-canvas sub-phase (Module architecture — shell; New-content detection).
+**Verify (in DAW):** Toggling a mode shows + plays only the active mode's managed
+lane, hides + silences the inactive-mode lane, and **never touches a manual lane**
+(its `C_LANEPLAYS` stays exactly as the user set it); new content created while a
+mode is active is tagged to that mode; pre-existing content stays Arrange (no
+mass-tag on the first poll after open).
+**Depends on:** D2-W1.
+
+- [ ] Apply managed-lane ops in the view shell (`I_FREEMODE`/`I_FIXEDLANE`/
+  `C_LANEPLAYS`/`B_FIXEDLANE_HIDDEN` via the item/track info setters;
+  `UpdateTimeline()` after `I_FREEMODE`); **managed lanes only, never manual**.
+  Verify every flag name/signature against the SDK header.
+- [ ] New-content detection on the bank_panel timer: diff the live track/item GUID
+  set against the previous poll; tag any GUID new since the last poll to the
+  then-active mode, with a **first-poll-after-open guard** (pre-existing ⇒ Arrange,
+  no mass-tag) and the **manual-lane exemption** (items in a manual lane not tagged).
+- [ ] Resolve the manual-lane detection heuristic (which new items are exempt) and
+  the `I_FIXEDLANE` lane-identity fragility (index survival across lane
+  reorder/renumber/deletion) — the two open design points from CONTEXT.md.
+- [ ] D2-W1 review polish: document the one-managed-lane-per-mode-per-track
+  exclusivity assumption in `laneModeState` (comment / debug-guard); clarify the
+  `serialize()` one-line style note; optional round-trip tests for the
+  last-writer-wins lane-replace contract.
+
+## D2-W3 — actions, persist wiring, panel UI
+**Goal:** Any new lane/mode-management actions, the persist slice serializing the
+lane-ownership index alongside the membership index, and any panel UI indicator.
+See CONTEXT.md §Two-canvas sub-phase (Module architecture — persistence).
+**Verify (in DAW):** The lane-ownership index survives Save / Save As / reopen
+(rides in the `"reasampler"` `view_state` alongside the membership index);
+lane/mode-management actions are registered and bindable.
+**Depends on:** D2-W2.
+
+- [ ] Any new lane/mode-management actions (`command_id`/`gaccel`/`hookcommand`);
+  bindable in the Actions list.
+- [ ] Persist slice: serialize/deserialize the lane-ownership index in the
+  `"reasampler"` `view_state` section, alongside the membership index.
+- [ ] Any panel UI indicator for lane/mode state.
+
+---
+
 # Phase B — Multi-bank (parallel to the M0–M11 capture roadmap and Phase D)
 
 > **Separate phase namespace.** The M-numbers belong to the capture pillar