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.

docs/COMPLETED.md

@@ -504,3 +504,51 @@ over the amp gain curve and persist it. Fixed on both routes (the popup close no
 cancels the drag; the mutable accessor refuses `kNone`). It has **no automated
 regression pin** — `src/shell/instrument/` has no test target, and the bug is shell
 state-machine coupling with no pure-layer equivalent.
+
+### Θ-W5-T1 — spline-egs
+
+Ships a free-drawn alternative to every staged envelope: the pitch, filter, and amp EGs
+can each switch Staged → Spline and have their contour drawn directly on the waveform
+overlay. The one shared monotone-spline implementation
+(`core/instrument/engine/velocity_curve`) gained **hard points** as a per-segment rule —
+a hard point does no curve smoothing on either adjacent segment, so the natural sharp
+angle stands instead of a continuous derivative — and the enhancement flows to every
+consumer, including the existing velocity→amp transfer curve, with no fork.
+
+- **Dual state, save-but-inactive.** Both the Staged and Spline state persist
+  simultaneously; switching modes never converts or discards the inactive one, so
+  Staged↔Spline round-trips losslessly. Params payload reached **v13**; v12 projects
+  still load.
+- **Gate unavailable in Spline mode.** A Spline EG's contour always covers the full
+  sample length as a pure time function (the Trigger/one-shot playback model), so Gate
+  is not selectable while it's active.
+- **Point-editing grammar converged**: left-click adds a point, right-click deletes it,
+  control-click toggles hard/smooth — one grammar shared by both spline consumers (the
+  EG overlay and the velocity-curve popup), matching the popup's already-shipped
+  right-click delete.
+- **Point-count ceiling: 128 — a musical bound, not a performance one.** Segment lookup
+  is an indexed binary search (≤7 steps at 128 points); the cap exists so long rhythmic
+  phrases (roughly two points per articulation event) aren't limited, not because the
+  evaluator is expensive.
+- **Staged controls disabled while Spline is active** — that envelope's segment knobs
+  and their inner curve dials render disabled and reject edits; the dormant staged state
+  is edited only by switching back to Staged.
+- The overlay's contour is normalized to the full sample length and drawn 1:1 with the
+  sample's time axis; a different-length capture rescales the stored contour
+  proportionally.
+
+A follow-on change in the same track reworked deck cell width: `-1` in `cellIds` changed
+meaning from "a blank cell holding geometry" to **one cell's width, reserved and
+redistributed** — a Trigger face that drops Sustain and Release now gets wider cells
+instead of 144 px of dead slots. Group widths, row packing, deck height, and Gate-mode
+cell widths are unchanged.
+
+**Deviations from spec / code review:**
+- A pure `resolveWaveformClaim` predicate (`core/instrument/ui/spline_edit`) now resolves
+  competing waveform-band clicks — contour node, crossfade tab, marker column, staged
+  envelope node — by **smallest nominal target area among candidates that actually
+  contain the click**, replacing resolution by check order.
+- The Gate-unavailable-while-drawn rule was consolidated into
+  `enforceGateUnavailableWhileDrawn` (`core/instrument/engine/play_params.h`), now the
+  single home of that rule, called by both `resolvePlay` and the editor's
+  `applyControl`.