Archive Milestone 4 in docs

Move completed M4 (persist) from PLAN.md to COMPLETED.md with a decisions
note (GUID identity, Save-As copy, MarkProjectDirty, known limitation);
update CLAUDE.md repo-identity to M0-M4 complete.

PLAN.md

@@ -14,18 +14,6 @@ it here and appends it to `COMPLETED.md`.
 
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-## Milestone 4 — persist (index ↔ project ext state)
-**Goal:** Write the `BankIndex` JSON to project ext state, reload on project open;
-project-relative path resolution. CONTEXT.md §persist, §Persistence & paths.
-**Verify (in DAW):** Index survives Save / Save As / close+reopen; **bank travels
-with the .rpp**; **relative paths only** in the persisted index (Save As to a new
-folder still resolves the bank).
-
-- [ ] `SetProjExtState` / `GetProjExtState` under namespace `"reasampler"`.
-- [ ] Bank-folder resolution from the current project path
-  (`EnumProjects` / `GetProjectPathEx`); store under a project-relative subfolder.
-- [ ] Reload-on-open; confirm survival across Save / Save As.
-
 ## 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