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.

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