Studio: re-expose keyframe retiming via a UI gesture (reducer already supports it, just unwired since #1763) #1782

Summary

Since #1763 removed the timeline keyframe-drag affordance, there is no GUI gesture left to retime an existing keyframe in time while preserving its value and easing. The only ways to move a keyframe in time via the UI now (delete + re-add at playhead, toggle-at-playhead) bake the current computed values and drop the explicit ease — so the author loses the keyframe they were trying to move.

The interesting part: the capability to retime-while-preserving still exists in the reducer — it's just not wired to any UI action anymore.

What still exists vs. what's reachable

Reading the served Studio bundle (dist/studio/assets/index-*.js, v0.7.18), the reducer handles a setGsapKeyframe action that does exactly the right thing:

case "setGsapKeyframe":
  return nAe(t, e.animationId, e.keyframeIndex, e.position, e.value, e.ease);
// nAe: position = e.position ?? current ; value = e.value ?? {...existing.properties} ; ease = e.ease ?? existing.ease
//  → passing ONLY a new `position` retimes the keyframe while keeping its value AND ease.

But setGsapKeyframe appears only twice in the whole bundle — the reducer case above and the validation switch. No UI code dispatches { type: "setGsapKeyframe", position: … }. The keyframe diamond now binds only onClick (select + seek) and onContextMenu, and the context menu wires only "change ease" and "delete" — there is no "set time / move to playhead" action. So the preserving-retime path is reachable from the SDK/code but dead from the GUI.

Request

Re-expose keyframe retiming through a UI gesture by dispatching the existing setGsapKeyframe with a new position (preserving value + ease). A drag affordance isn't required — a menu entry is enough. For example, add to the keyframe context menu:

• "Move keyframe to playhead" → setGsapKeyframe({ animationId, keyframeIndex, position: playheadPercent }), and/or
• an editable time / percentage field for the selected keyframe.

This reuses code that's already present; it's a wiring task, not new engine work.

Use case / motivation

Our workflow has a non-technical author posing keyframes by gesture in the Studio, then a technical editor refining in source. Retiming is currently the one operation that can't be done at the gesture level without destroying the keyframe — so every retime has to round-trip through hand-editing tl.to(sel, { keyframes: { … } }) in code. Restoring a gesture that retimes-in-place (preserving value + ease) would let the author iterate on timing directly, which is the common case.

Environment

• hyperframes CLI / Studio 0.7.18 (latest)
• Related: fix(studio): remove keyframe dragging from the timeline #1763 (removed the previous timeline keyframe-drag affordance)