Build your own easing curve and use it on keys and Lerp (#247)

The built-in eases cover the usual shapes, but a game often wants its
own: a coin that bounces twice, a door that overshoots and settles.
A new asset type holds such a curve, and keys and the Lerp node can
pick it instead of a built-in ease.

- New asset type "curve": points like in Vector art, x is t and y is
  the value. A new curve is the straight line from (0, 0) to (1, 1).
- One pure evaluation in src/core/model/curve.ts serves the timeline
  preview, the running game and the Lerp node.
- The curve always has one value per t. The end points stay at t 0
  and 1, points cannot pass each other, and a handle that reaches past
  its neighbour is shortened along its direction. Loading a hand-edited
  file applies the same rules.
- Curve tab: click on the line to insert a point that keeps the shape,
  click on free space to put one there, drag points and handles,
  double-click or Delete removes, Alt-click toggles round and sharp.
  Every step is one undo step.
- Presets: all 30 built-in eases plus Linear, fitted with adaptive
  subdivision. Each stays within 0.4 % of the built-in ease.
- Preview: a dot runs along the curve once a second, a second dot
  shows only the value. The Preview button stops it.
- Animation keys get "Curve" under Easing with a curve picker. Lerp
  gets "Curve" under Ease, and its result may overshoot b.
- A missing or unpicked curve runs linear. The inspector says so, the
  node shows it as a problem, and deleting a used curve warns with the
  number of keys and Lerp nodes.

Tests:
- ./check.sh: 180 files, 2612 tests, CHECK EXIT 0
- GM_STEPS=1-6,25,53: ALL PASSED, 309 checks
- New e2e module 53 with 13 checks, all with the real mouse. Expected
  values come from its own Bezier evaluation.
- Counter-checks bite: without Bezier evaluation 4 unit tests fail,
  without handle clamping 2. In the browser, a key that cannot find its
  curve, a click that sets no point and a handle drag that moves the
  point all turn checks red.

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

src/editor/state.ts

@@ -645,6 +645,13 @@ export class EditorState {
     return findAssetOf(this.project, s.assetId, 'tile')?.id ?? null;
   }
 
+  /** The curve the selection refers to (#247); only the asset itself, its points are picked in the tab. */
+  get selectedCurveId(): string | null {
+    const s = this.selection;
+    if (s?.kind !== 'asset') return null;
+    return findAssetOf(this.project, s.assetId, 'curve')?.id ?? null;
+  }
+
   /** The timeline row the selection is about; undefined when it is about something else. */
   get selectedTrackId(): string | undefined {
     return this.selection?.kind === 'track' ? this.selection.trackId : undefined;