Offer the easings of an animation key on Lerp and sound FX ramps

A timed Set sound FX parameters ran only linearly, and a Lerp had four
quadratic eases of its own. An animation key already had the full set.
Now all three offer the same choice: Linear, Step, the ten curves from
Sine to Bounce with In, Out or In-out, and a drawn Curve asset.

- easingParams in src/core/graph/nodes/easing.ts gives a node the
  fields Easing, Direction and Curve from the key's own lists, and
  easingOfParams reads them back as a key interpolation.
- There is no migration. A Lerp saved with the old in, out or inOut
  runs linear until its easing is picked again.
- Set sound FX parameters hands the easing to the player. Linear and
  Step stay ramps of the audio clock, every other easing goes to
  setValueCurveAtTime as 200 samples a second, in semitones and dB.
- Step now reaches its end at t 1. A key never asked for that, it
  reads the next key itself, so animations play as before.
- The curve-in-use count and the warning for a gone curve cover the
  sound node too.

Tests:
- node-catalog: Lerp with cubic in, quad out and in-out, a curve
  without a direction, step, an unknown easing as linear. The sound
  node hands on a curve, step and a gone curve as linear.
- sound-library: an eased change becomes a curve in semitones and
  decibels, a step a jump at the end.
- node-catalog and curve after dropping the old values: 399, green.
- e2e 6, 34, 53 on melektaus-atx: ALL PASSED, 198 checks. In Chrome a
  2 s Quad in from depth 7 to -5 read speed 0.864 early and 0.5 at the
  end, a 1 s Step held 1 and then jumped to 0.5.
- Counter-check: with curves sent as linear ramps, the curve test of
  the sound library fails.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/core/model/animation.ts

@@ -233,7 +233,8 @@ export function easedShare(
 ): number {
   const { shape, direction, curve } = easingOf(interpolation);
   if (shape === 'linear') return t;
-  if (shape === 'step') return 0;
+  // A key reads the next key's own value once it is there, so for it a step only ever holds.
+  if (shape === 'step') return t >= 1 ? 1 : 0;
   if (shape === 'curve') {
     const points = curves(curve ?? '');
     return points ? curveValue(points, t) : t;
@@ -249,6 +250,19 @@ export function easedShare(
   }
 }
 
+/**
+ * An easing as something that runs on its own clock, the audio one: the two shapes it has built in
+ * stay names, everything else is the share at each `t`, a gone curve already read as linear.
+ */
+export type EaseShare = 'linear' | 'step' | ((t: number) => number);
+
+export function easeShareOf(interpolation: KeyInterpolation, curves: CurveLookup = NO_CURVES): EaseShare {
+  const { shape, curve } = easingOf(interpolation);
+  if (shape === 'linear' || shape === 'step') return shape;
+  if (shape === 'curve' && !curves(curve ?? '')) return 'linear';
+  return (t) => easedShare(interpolation, t, curves);
+}
+
 /** The value between two keys, `t` from 0 (at `a`) to 1 (at `b`). */
 export function blendValue(kind: string, a: PropertyValue, b: PropertyValue, t: number): PropertyValue {
   if (kind === 'number') {

src/core/model/curve.ts

@@ -15,8 +15,10 @@ import { cloneVectorPoint } from './vectorart';
 /** The node type that eases along a curve, and its setting naming the curve. */
 export const LERP = 'Lerp';
 export const LERP_CURVE = 'curve';
+/** Every node type with the easing settings of a `Lerp`, the curve among them. */
+export const EASED_NODES: readonly string[] = [LERP, 'SetSoundFxParameters'];
 
-/** How many animation keys and `Lerp` nodes ease along a curve – what deleting it has to warn about. */
+/** How many animation keys and eased nodes run along a curve – what deleting it has to warn about. */
 export function curveUsages(project: Project, curveId: string): { keys: number; nodes: number } {
   let keys = 0;
   let nodes = 0;
@@ -25,7 +27,9 @@ export function curveUsages(project: Project, curveId: string): { keys: number;
       for (const track of asset.tracks)
         keys += track.keys.filter((key) => key.interpolation === `curve:${curveId}`).length;
     if (isGraphAsset(asset))
-      nodes += asset.graph.nodes.filter((n) => n.type === LERP && n.params[LERP_CURVE] === curveId).length;
+      nodes += asset.graph.nodes.filter(
+        (n) => EASED_NODES.includes(n.type) && n.params[LERP_CURVE] === curveId,
+      ).length;
   }
   return { keys, nodes };
 }