docs: close out Θ-W5, and record two spline-overlay UX deferrals

Collapse Θ-W5 to wave-level in PLAN.md, append its narrative to COMPLETED.md, fix a stale resolvePlay reference in core/instrument/CLAUDE.md, and file the drag-off-margin and AttackEnd/Origin shadow warts in TODO.md.

src/core/instrument/CLAUDE.md

@@ -230,8 +230,10 @@ consequences, each with one home:
 
 - **Gate is unavailable while any EG is drawn.** A contour is a pure time function over the full
   sample length, which IS the Trigger/one-shot model. `splineActive` (`play_params.h`) is the
-  predicate; `resolvePlay` enforces it on the way to the engine and the editor's Gate segment
-  refuses and paints Disabled off the same predicate.
+  predicate; `enforceGateUnavailableWhileDrawn` (`play_params.h`) is the one enforcement of it,
+  called by both `resolvePlay` (`sample_map.cpp`) on the way to the engine and the editor's
+  `applyControl`, so the two callers cannot drift. The editor's Gate segment refuses and paints
+  Disabled off the same `splineActive` predicate.
 - **A drawn envelope's staged segment knobs go inert** — drawn-but-dead, never removed, never
   hidden — including their inner curve dials, which are reached through their outer cell.
   `deckKnobInert` (`ui/deck_groups`) is the one place that list lives. The DEPTH knobs (pitch