docs(multi-bank): settle Phase B forks 1-4; draft fork-5 options analysis

Fold Daniel's decisions into CONTEXT.md, PLAN.md, and the product notes:
- Fork 1 (persistence): pool folds into the `banks` blob; legacy
  `bank_index` key retired after a one-way lossless migration.
- Fork 2 (delete): delete drops member index entries; add an `evacuate`
  verb (B1 pure op + B3 action) returning members to the pool. Design the
  orphaned-until-prune window and confirm-on-non-empty-delete guardrail.
- Fork 3 (move): move is the default gesture, copy the deliberate secondary.
- Fork 4 (active bank): active-bank and shown-tab stay distinct; the
  active-bank indicator must be visually unmistakable.

Fork 5 (tab rendering + move affordance) remains open with an options
analysis and pending recommendations. Additive; M0-M11 and Phase D untouched.

PLAN.md

@@ -98,42 +98,53 @@ privileged, create/rename/reorder/delete named banks, active-bank id, move/copy
 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 +
+**Verify:** CTest green. Pool always present, un-deletable, un-renamable,
+un-evacuable (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. Delete drops member index entries.
+Evacuate moves all members to the pool, leaving the bank empty. JSON round-trip
+lossless across pool-as-bank-zero + named 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.
+  seeded with fixed id + fixed name; create / rename / reorder / delete named banks
+  (delete drops the bank's member index entries).
 - [ ] Pool-privilege rules enforced in-model: reject delete-pool, reject
-  rename-pool, never allow zero banks.
+  rename-pool, reject evacuate-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.
+- [ ] Evacuate bank: move every member to the pool (index-only; destination
+  collapse-by-hash observed), leaving the bank empty; pool cannot be evacuated.
+- [ ] JSON round-trip: pool-as-bank-zero inside the blob + named 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.
+  zero named banks (one-way, lossless; blob authoritative thereafter).
+- [ ] Tests: pool privileges (delete/rename/evacuate rejected); move
+  source-loses/dest-gains; copy source-retained; evacuate empties source into pool
+  with dest collapse; 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).
+existing sections, with the pool folded in as bank-zero; migrate a legacy
+`bank_index` key into the pool on first load and retire the legacy key; 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, and after save carries `banks` with no `bank_index`
+written.
+**Depends on:** B1. (Persistence-key fork settled — fork 1 (a): pool inside the
+`banks` blob, legacy key retired after one-way migration.)
 
-- [ ] 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.
+- [ ] Serialize/deserialize the book under the `banks` key (pool-as-bank-zero inside
+  the blob; distinct section from `view_state`; no `bank_index` key written going
+  forward).
+- [ ] Legacy-migration path on load: absent `banks` + present `bank_index` → promote
+  into pool, mint the blob, treat blob as authoritative (legacy key retired).
 - [ ] 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.
@@ -141,44 +152,54 @@ only) loads as pool + zero named banks with no sample loss.
 ## 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.
+bank-activate + move/copy + evacuate MIDI-bindable; create/rename/delete/evacuate
+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).
+- [ ] Create bank / rename bank / delete bank (delete drops member index entries;
+  confirm-on-non-empty offered at the UI layer in B4).
+- [ ] Evacuate bank → pool (move all members back to the pool; refuses on the pool).
 - [ ] Activate bank (direct-by-id + cycle).
-- [ ] Move selected samples → bank / copy selected samples → bank.
+- [ ] Move selected samples → bank / copy selected samples → bank (move is default).
 - [ ] Pool full-height / banks full-height toggles.
 - [ ] Register each (`command_id`/`gaccel`/`hookcommand`); bank-activate + move/copy
-  MIDI-bindable.
+  + evacuate 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.
+below (empty when no named banks, one tab per named bank); active-bank **unmistakably**
+indicated; both full-height toggles collapse the split correctly; sample move/copy
+affordance works; non-empty delete confirms and offers evacuate; the Design View mode
+switch in the header is unaffected.
+**Depends on:** B1, B2, B3. (Tab rendering + move-affordance mechanics — fork 5 —
+pending Daniel's decision; see Phase B open questions and product notes → *Fork 5*.)
 
 - [ ] 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.
+  (SWELL-native vs. LICE-drawn — fork 5a, pending.)
 - [ ] 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).
+- [ ] Active-bank indicator — **visually unmistakable** (settled constraint);
+  placement (per-region header / single readout / lit-tab) is the residual polish
+  detail.
+- [ ] Create / rename / delete / activate / evacuate affordances driving B3 actions.
+- [ ] Delete confirms on a non-empty bank, naming the evacuate alternative.
+- [ ] Sample move/copy affordance (drag between regions and/or "send to bank" menu —
+  fork 5b, pending).
 
 ## 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.
+Forks 1–4 settled by Daniel (2026-07-23): persistence key = fold pool into `banks`,
+retire legacy key (1a); delete drops members + add evacuate verb (2); move is the
+default gesture (3); active-bank/shown-tab distinct with an unmistakable indicator
+(4). Folded into CONTEXT.md §Multi-bank + the B1–B4 points above. Remaining:
+
+- **Fork 5 — tab rendering + move affordance** — (5a) SWELL-native tab control vs.
+  LICE-drawn tabs matching the grid aesthetic; (5b) drag-between-regions vs.
+  "send to bank" menu vs. both. Options analysis + pending recommendations in product
+  notes → *Fork 5*. Verify SWELL tab-control availability + cross-platform parity
+  against the SWELL headers / SWS reference before choosing native tabs. Decision
+  pending Daniel. (touches B4)
+- **Active-bank indicator placement** — per-region headers vs. single header readout
+  vs. lit-tab. "Unmistakable" is settled; only placement is open. Polish detail.
   (touches B4)
-- **Active-bank indicator placement** — per-region headers vs. single header
-  readout. (touches B4)