Merge Γ-W2-T2: an explicit loop enable, four named marks with grabbable caps, and the crossfade painted where it is heard

docs/TODO.md

@@ -172,13 +172,7 @@ Forward-looking follow-ups. Deferred by decision, not oversight — each entry r
 
 **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.
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-**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.
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-**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.
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-**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.
+**RESOLVED — Γ-W2-T2 (`loop-crossfade-ux`), incidentally.** Giving every mark the cap-grip the crossfade already had is what closed it: the start marker now carries an 11x10 cap in the overlay's top strip, whose nominal area (110) is smaller than the node's fixed pick box (169), so the cap wins the coincident pixel and the marker is reachable again. No priority rule was added and `resolveWaveformClaim` is byte-for-byte unchanged — but the cap slot's own nominal area DID move, from the old clipped-actual measure (60 at frame 0) to the new nominal 110 every cap now feeds it (`markerHandleRect`'s own unclipped area). That move leaves the `cap < node < column` ordering unchanged only because 110 is still under the node's fixed 169 — the outcome held, not the area. Below the cap strip the node keeps the click, which is correct: that is where the node is actually drawn for any non-degenerate envelope. Pinned by `testAMarkCapOutranksACoincidentEnvelopeNodeInTheTopStrip` (`tests/test_spline_edit.cpp`). `Origin` was not touched and `isDraggable`'s shape rules are unchanged.
 
 ## Active-bank indicator placement (B4 polish)