Archive Milestone 5; correct M4 identity note to GUID-primary

Move completed M5 (bank_panel: thumbnails, audition, multi-select, keyboard)
to COMPLETED.md and close the thumbnail-cache open question; reword the M4
Save-As identity note to the final GUID-primary/pointer-secondary design.

COMPLETED.md

@@ -114,7 +114,25 @@ folder still resolves the bank).
 - Storage: `SetProjExtState` / `GetProjExtState`, namespace `"reasampler"`, keys `bank_index` (serialized JSON) and `project_guid`; relative paths only in the persisted index.
 - Project identity: keyed off a **minted GUID** stored in ext state (REAPER exposes no native per-project GUID), not the raw `ReaProject*` — a recycled pointer cannot misread a project switch as a Save-As.
 - Save-As: **copy** semantics (Daniel's decision) — the `reasampler_bank/` folder is copied under the new `.rpp`; the old project's bank stays intact. Every ext-state write calls `MarkProjectDirty` so captures/GUID changes flush on the normal save.
-- Save-As collision — fixed (DAW-verified): the project-identity classifier now keys off the **`ReaProject*` object identity** (`sameProjectObject` signal), not the stored GUID alone. A different project object (tab-switch or open) is always treated as a Load and never relocates a bank, so two Save-As-forked projects sharing a copied GUID on disk cannot cross-contaminate. A forked sibling that still shares the original GUID is re-GUID'd on detection so identities diverge. The hole is closed without force-saving — non-destructive preserved.
+- Save-As collision — fixed (DAW-verified): project identity is **GUID-primary** — the stored per-project GUID is the identity of record; a different stored GUID always means a different project (Load its bank), immune to REAPER recycling `ReaProject*` addresses across close/open. The `ReaProject*` pointer is a secondary signal that disambiguates the same-GUID case only: a different object with the same GUID = a Save-As fork (Load + re-GUID to diverge); the same object with the same GUID + a new path = a genuine Save-As in progress (relocate bank). This replaced two earlier iterations: GUID-only (mis-detected forks sharing a copied GUID) and pointer-primary (mis-detected reopen/new-project because it ignored the GUID on address recycling). Non-destructive preserved.
+
+---
+
+## Milestone 5 — bank_panel (docked grid)
+**Goal:** Docked LICE-drawn grid: thumbnails (from `peaks`), audition,
+multi-select, keyboard navigation. Reuses the docking setup from the retired
+`mpe_view.cpp`. CONTEXT.md §bank_panel.
+**Verify (in DAW):** Grid docks; thumbnails render from computed peaks; audition
+plays selected sample; multi-select + keyboard nav work.
+
+- [x] Docked window + LICE grid render loop.
+- [x] Thumbnail draw from `peaks` bins.
+- [x] Audition (play selected sample) + stop.
+- [x] Multi-select + keyboard navigation.
+
+**Notes/decisions:**
+- Thumbnail cache: in-memory recompute keyed by `(sampleId, drawWidth, bankGeneration)`; peak bins are NOT persisted alongside the index. Cache is discarded on bank change and rebuilt on next draw. (Closes the PLAN "thumbnail cache" open question.)
+- Audition: stock `PlayPreview` / `StopPreview` API, read-only — display + select + audition only, never inserts into the arrange. Single stop-funnel ensures a leak-free preview lifecycle. Flagged undocumented assumption: `StopPreview` detaches the source before returning; mitigated by the single-funnel design. Escalation path if a runtime pop appears: switch to `StartPreviewFade` + deferred free.
 
 ---
 

PLAN.md

@@ -14,18 +14,6 @@ it here and appends it to `COMPLETED.md`.
 
 ---
 
-## Milestone 5 — bank_panel (docked grid)
-**Goal:** Docked LICE-drawn grid: thumbnails (from `peaks`), audition,
-multi-select, keyboard navigation. Reuses the docking setup from the retired
-`mpe_view.cpp`. CONTEXT.md §bank_panel.
-**Verify (in DAW):** Grid docks; thumbnails render from computed peaks; audition
-plays selected sample; multi-select + keyboard nav work.
-
-- [ ] Docked window + LICE grid render loop.
-- [ ] Thumbnail draw from `peaks` bins.
-- [ ] Audition (play selected sample) + stop.
-- [ ] Multi-select + keyboard navigation.
-
 ## Milestone 6 — insert (placement)
 **Goal:** "Insert selected sample at edit cursor" via `InsertMedia`. CONTEXT.md
 §insert, Build order 6.
@@ -108,8 +96,6 @@ landed milestone.
 
 - **Realtime wet-master routing** that captures master output without altering the
   user's monitoring. (blocks M8)
-- **Thumbnail cache:** recompute peaks vs store peak bins alongside the index.
-  (touches M2/M4/M5)
 - **`parseInt` narrowing hardening:** `src/bank_model.cpp` `parseInt` casts
   `int64_t → int` via `static_cast` without a range check; integers that fit
   in int64 but exceed `INT_MAX` are implementation-defined. Hardening candidate