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/Editor.ts

@@ -70,6 +70,7 @@ import type { EditorState } from './state';
 import { copySelection } from './clipboard';
 import { PixelArtEditor } from './pixelart/PixelArtEditor';
 import { VectorArtEditor } from './vectorart/VectorArtEditor';
+import { CurveEditor } from './curve/CurveEditor';
 import { TextureEditor } from './texture/TextureEditor';
 import { installFileImport } from './fileImport';
 import type { PluginManager } from './plugins/loader';
@@ -88,6 +89,7 @@ export type CentreTab =
   | 'vectorart'
   | 'pixelart'
   | 'animation'
+  | 'curve'
   | 'tile'
   | 'container'
   | 'list'
@@ -134,6 +136,7 @@ const PANE_ORDER = [
   'vectorart',
   'pixelart',
   'animation',
+  'curve',
   'tile',
   'container',
   'list',
@@ -152,6 +155,7 @@ const TAB_LABELS: Record<CentreTab, string> = {
   vectorart: 'Vector art',
   pixelart: 'Pixel art',
   animation: 'Animation',
+  curve: 'Curve',
   tile: 'Tile',
   container: 'Container',
   list: 'List',
@@ -197,6 +201,7 @@ export class Editor implements PlayHost {
   readonly vectorArtEditor: VectorArtEditor;
   readonly pixelArtEditor: PixelArtEditor;
   readonly animationEditor: AnimationEditor;
+  readonly curveEditor: CurveEditor;
   readonly tileEditor: TileEditor;
   readonly containerEditor: ContainerEditor;
   readonly listEditor: ListEditor;
@@ -276,6 +281,7 @@ export class Editor implements PlayHost {
     this.vectorArtEditor = new VectorArtEditor(state);
     this.pixelArtEditor = new PixelArtEditor(state);
     this.animationEditor = new AnimationEditor(state);
+    this.curveEditor = new CurveEditor(state);
     this.tileEditor = new TileEditor(state);
     this.containerEditor = new ContainerEditor(state);
     this.listEditor = new ListEditor(state);
@@ -380,6 +386,7 @@ export class Editor implements PlayHost {
       vectorart: this.vectorArtEditor,
       pixelart: this.pixelArtEditor,
       animation: this.animationEditor,
+      curve: this.curveEditor,
       tile: this.tileEditor,
       container: this.containerEditor,
     };
@@ -814,6 +821,8 @@ export class Editor implements PlayHost {
     this.console.refresh();
     if (this.tab === 'animation') this.animationEditor.tick(step);
     if (this.tab === 'tile') this.tileEditor.tick(step);
+    // The preview is the editor's own, so it runs on while the game is paused.
+    if (this.tab === 'curve') this.curveEditor.tick(dt);
     // Play repaints the machine tab itself (see `PlaySession.tick`); an editor run has no session
     // to do it, and its states move just the same.
     if (this.tab === 'statemachine' && this.editorRun.isRunning) this.stateMachineEditor.canvas.redraw();
@@ -837,7 +846,8 @@ export class Editor implements PlayHost {
       this.tab === 'pixelart' ||
       (this.tab === 'shader' && this.shaderEditor.animates) ||
       (this.tab === 'animation' && this.animationEditor.isPlaying) ||
-      (this.tab === 'tile' && this.tileEditor.isPlaying)
+      (this.tab === 'tile' && this.tileEditor.isPlaying) ||
+      (this.tab === 'curve' && this.curveEditor.isPlaying)
     );
   }