docs(multi-bank): frame Phase B — pool + named banks spec

Add the multi-bank pillar as Phase B (lettered, parallel to M-line and
Phase D): CONTEXT.md §Multi-bank spec, PLAN.md Phase B block (B1 bank_book
pure / B2 persist / B3 actions / B4 panel split), and docs/product/multi-bank.md
framing. Additive only — BankIndex, the M0–M11 roadmap, and Phase D untouched.

PLAN.md

@@ -79,3 +79,106 @@ landed milestone.
   in int64 but exceed `INT_MAX` are implementation-defined. Hardening candidate
   — add bounds check before the cast when integer-field validation is in scope.
 
+---
+
+# 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
+> (M0–M11); the D-letters belong to Design View. Multi-bank is a third orthogonal
+> pillar — generalizing the single bank into a pool + named banks — so it takes its
+> own **lettered** namespace (B1, B2, …). "B" reads for **Banks** and, like Phase D,
+> keeps the roadmaps from colliding on numbering: Phase B is not "the twelfth
+> capture step," it is a different pillar. Authoritative spec: **CONTEXT.md
+> §Multi-bank**. Product framing: `docs/product/multi-bank.md`. When a point lands,
+> doc-keeper moves it to `COMPLETED.md`.
+
+## B1 — bank_book (pure)
+**Goal:** REAPER-free bank registry wrapping N `BankIndex` instances: pool seeded +
+privileged, create/rename/reorder/delete named banks, active-bank id, move/copy a
+sample between banks, JSON round-trip + legacy-migration. The heart of the phase;
+mirror of `bank_model` / `view_mode_model`; **`BankIndex` untouched (additive)**.
+CONTEXT.md §Multi-bank (Module architecture — pure).
+**Verify:** CTest green. Pool always present, un-deletable, un-renamable (rules
+rejected in-model). Active-bank defaults to pool. Move is index-only (source loses
+entry, destination gains it) and observes destination collapse-by-hash; copy leaves
+source intact. JSON round-trip lossless across banks + per-bank indices + ordinals +
+active id. Legacy `bank_index` JSON parses into `{ pool }` with zero named banks.
+
+- [ ] Bank registry: ordered `{ bank id, display name, ordinal, BankIndex }`; pool
+  seeded with fixed id + fixed name; create / rename / reorder / delete named banks.
+- [ ] Pool-privilege rules enforced in-model: reject delete-pool, reject
+  rename-pool, never allow zero banks.
+- [ ] Active-bank id (get/set; defaults to pool); resolve active bank's `BankIndex`.
+- [ ] Move sample between banks (index-only; destination collapse-by-hash observed;
+  source entry removed).
+- [ ] Copy sample between banks (index-only; source entry retained; destination
+  collapse-by-hash observed).
+- [ ] JSON round-trip: banks + per-bank indices + ordinals + active id.
+- [ ] Legacy migration: a bare `bank_index` JSON promotes to the pool's index with
+  zero named banks (one-way, lossless).
+- [ ] Tests: pool privileges (delete/rename rejected); move source-loses/dest-gains;
+  copy source-retained; cross-bank same-hash coexistence; dest collapse on move into
+  a bank already holding the hash; JSON lossless; legacy migration.
+
+## B2 — persist slice (banks ↔ project ext state)
+**Goal:** Serialize the book under the `banks` key in `"reasampler"` alongside the
+existing sections; migrate a legacy `bank_index` key into the pool on first load;
+reload-on-open and Save-As survival via the existing M4 machinery. CONTEXT.md
+§Multi-bank (persist). **Verify (in DAW):** Banks + named banks + active bank + all
+per-bank samples survive Save / Save As / close+reopen; **relative paths only**;
+bank travels with the `.rpp`; a project saved before this phase (legacy `bank_index`
+only) loads as pool + zero named banks with no sample loss.
+**Depends on:** B1. Resolve the `bank_index` retirement-vs-retention fork first
+(see Phase B open questions).
+
+- [ ] Serialize/deserialize the book under the `banks` key (shared blob, distinct
+  section from `bank_index` / `view_state`).
+- [ ] Legacy-migration path on load: absent `banks` + present `bank_index` → pool.
+- [ ] Session exposes the book; the active bank's `BankIndex` is the capture add
+  target (route the M7 capture family through it — additive to M7, no M7 rewrite).
+- [ ] Confirm survival across Save / Save As; confirm legacy-project load path.
+
+## B3 — actions
+**Goal:** Bindable action set for the multi-bank workflow. CONTEXT.md §Multi-bank
+(actions). **Verify (in DAW):** Each action registered (bindable in Actions list);
+bank-activate + move/copy MIDI-bindable; create/rename/delete drive the B1 model via
+the B2-persisted session.
+**Depends on:** B1, B2.
+
+- [ ] Create bank / rename bank / delete bank (delete honors the resolved
+  member-disposition rule — see open questions).
+- [ ] Activate bank (direct-by-id + cycle).
+- [ ] Move selected samples → bank / copy selected samples → bank.
+- [ ] Pool full-height / banks full-height toggles.
+- [ ] Register each (`command_id`/`gaccel`/`hookcommand`); bank-activate + move/copy
+  MIDI-bindable.
+
+## B4 — bank_panel vertical split (UI)
+**Goal:** The vertical-split bank window — pool on top, named-banks tab-page region
+below, full-height toggles — extending the M5 docked grid. CONTEXT.md §Multi-bank
+(bank_panel). **Verify (in DAW):** Pool grid renders on top; named-banks tab strip
+below (empty when no named banks, one tab per named bank); active-bank indicated;
+both full-height toggles collapse the split correctly; sample move/copy affordance
+works (drag and/or menu); the Design View mode switch in the header is unaffected.
+**Depends on:** B1, B2, B3.
+
+- [ ] Vertical split: pool grid region (top) + named-banks tab-page region (bottom).
+- [ ] Named-banks tab strip: one tab per named bank; empty state when none.
+- [ ] Pool full-height / banks full-height toggle affordances wired to B3.
+- [ ] Active-bank indicator.
+- [ ] Create / rename / delete / activate affordances driving B3 actions.
+- [ ] Sample move/copy affordance (drag between regions and/or "send to bank" menu).
+
+## Phase B open questions
+- **`bank_index` key retirement vs. retention** — (a) fold the pool into the `banks`
+  blob and retire the legacy key, vs (b) keep `bank_index` as the pool's canonical
+  slot and store only named banks under `banks`. Leaning (b) for additive-minimalism
+  against in-flight M7/M8 persist work. Resolve before B2. (blocks B2)
+- **Named-bank delete → member disposition** — (i) reabsorb into pool, (ii)
+  orphan-check by file reference, (iii) forbid non-empty delete. Leaning (i).
+  (touches B1/B3)
+- **Move affordance + tab rendering** — drag vs. menu as primary; SWELL-native vs.
+  LICE-drawn tabs. Verify SWELL tab-control availability against the M5 reference.
+  (touches B4)
+- **Active-bank indicator placement** — per-region headers vs. single header
+  readout. (touches B4)