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/TODO.md

@@ -152,6 +152,34 @@ Forward-looking follow-ups. Deferred by decision, not oversight — each entry r
 
 **Done looks like.** Not stated in the source beyond "confirm no surprising fight."
 
+## Spline overlay's drag-off delete margin may be too generous for its box
+
+**Context (what shipped — Θ-W5-T1, spline-egs).** `kCurveDragOffMargin = 24` (`editor_internal.h`) was sized for the velocity-curve popup, whose editing box floats with slack on all sides — the popup's own sheet border sits well outside the box, so 24px of overshoot before a drag-off delete arms is comfortably inside the sheet. The Spline EG overlay reuses the same constant and the same drag-off-delete logic verbatim (`editor_paint_waveform.cpp`), but its box abuts the deck directly with no equivalent slack.
+
+**The wart.** Dragging an overlay contour node toward the bottom of the waveform band and overshooting roughly 24px past the box floor carries the drag into the deck below and arms a delete — a gesture that reads as "drag toward the deck" rather than "delete this point." Mitigations already in place: a WARN paint cue while the drag is armed-to-delete, and `VelocityCurve::deletePoint` unconditionally refuses the two endpoints regardless of margin.
+
+**Intended fix.** Not yet proposed — likely a smaller, overlay-specific margin (or a margin derived from the actual gap between the overlay box and the deck) rather than sharing the popup's constant.
+
+**The constraint the fix MUST handle.** Whatever margin the overlay uses must still comfortably permit an intentional delete-by-drag-off gesture (the design's stated point-removal path) without shrinking it into a hair-trigger; the popup's own margin and delete behavior must be left untouched.
+
+**Priority / risk.** Low, pending Daniel's hands-on assessment. Flagged by code review as an unmeasured UX judgment, not a confirmed defect — whether the overshoot is a real hazard in practice is Daniel's call.
+
+**Done looks like.** Daniel has used the Spline EG overlay hands-on and either confirms the current margin is fine as shared, or a separate overlay margin is chosen and the WARN cue's trigger point is verified to match it.
+
+## Pre-existing staged-envelope-node shadow at zero-attack (AttackEnd on Origin)
+
+**Context (what shipped — Θ-W5-T1, spline-egs).** The staged envelope-node hit-test (`nodeAtPoint`, `envelope_edit.cpp`) and the drawn contour's node hit-test now feed the SAME `WaveformClaim` arbitration slot in `resolveWaveformClaim` (`spline_edit.h`), which resolves competing waveform-band claims — node, crossfade tab, marker column — by smallest nominal target area among the candidates that actually hit. This is the same defect class as the contour-node/marker collision W5 fixed by replacing check-order resolution with that arbitration.
+
+**The wart.** A zero-attack `AttackEnd` vertex is drawn at the same pixel as `Origin` (the envelope's non-draggable start anchor), which for an AHD envelope sits at the start marker's frame. Because a node's nominal pick-box area is smaller than the marker's full-height grab-column area, and `resolveWaveformClaim`'s rule is "smallest area among hit candidates wins," the draggable `AttackEnd` node still claims the click over the start marker when the two coincide — and, at a loop starting there, over the crossfade tab. Folding the staged pass into the shared arbitration slot did not change this specific outcome, since the rule that decides node-vs-marker priority is unchanged from what the contour-node fix established. `Origin` itself is excluded from `nodeAtPoint`'s candidate set entirely (never draggable, never a hit), so the common case — attack > 0, no coincidence — is unaffected.
+
+**Intended fix.** Not yet proposed. Bringing the staged pass into the shared arbitration slot was the natural first step and has landed; closing the remaining collision needs either a per-affordance priority rule for genuinely coincident precision targets, or accepting the current smallest-area outcome as intended and documenting it as such rather than as an open wart.
+
+**The constraint the fix MUST handle.** Whatever rule changes must not regress the contour-node/marker and tab/marker arbitration W5 already fixed, and must not make `Origin` draggable or otherwise touch `isDraggable`'s AHD/AHDSR shape rules.
+
+**Priority / risk.** Low. Pre-existing, not introduced by W5; the common case (nonzero attack) is unaffected, and the collision requires both a zero-attack stage and a coincident marker/tab to be reachable at all.
+
+**Done looks like.** A zero-attack `AttackEnd` node coincident with the start marker (or, on a loop starting there, the crossfade tab) no longer silently claims the click ahead of the marker/tab — either by an explicit priority rule or by a recorded decision that the current behavior is intended.
+
 ## Active-bank indicator placement (B4 polish)
 
 **Context.** CONTEXT-ARCHIVE.md's "Open questions to resolve during build" (B4 panel section): forks 1–5 are all settled; one panel-polish detail remains open. Fork 4 already settled that the active-bank indicator must be "visually unmistakable" — only its placement is undecided.