Archive Milestone 6 in docs

Move completed M6 (insert placement onto selected tracks at edit cursor)
from PLAN.md to COMPLETED.md with a decisions note.

PLAN.md

@@ -14,17 +14,6 @@ it here and appends it to `COMPLETED.md`.
 
 ---
 
-## Milestone 6 — insert (placement)
-**Goal:** "Insert selected sample at edit cursor" via `InsertMedia`. CONTEXT.md
-§insert, Build order 6.
-**Verify (in DAW):** Selected sample inserts at the edit cursor wrapped in
-`Undo_BeginBlock2` / `Undo_EndBlock2`; conform-to-tempo is an explicit flag —
-**no silent time-stretch** when off.
-
-- [ ] `InsertMedia(path, mode)` at edit cursor (verify mode bits against SDK).
-- [ ] Conform-to-project-tempo vs literal as an explicit flag (never silent).
-- [ ] Undo-block wrapping.
-
 ## Milestone 7 — capture action family
 **Goal:** Bindable capture actions for master / selected tracks / selected items /
 razor area, each with wet-dry + tail options. CONTEXT.md §actions, Build order 7.