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/panels/assetGroups.ts

@@ -23,6 +23,7 @@ export const ASSET_GROUP_LABELS: Record<AssetType, string> = {
   pixelart: 'Pixel drawings',
   palette: 'Palettes',
   animation: 'Animations',
+  curve: 'Curves',
   tile: 'Tiles',
   tilemap: 'Tilemaps',
   statemachine: 'State machines',

src/editor/panels/assetIcons.ts

@@ -8,6 +8,7 @@ import behavior from './icons/behavior.svg?raw';
 import camera from './icons/camera.svg?raw';
 import collider from './icons/collider.svg?raw';
 import container from './icons/container.svg?raw';
+import curve from './icons/curve.svg?raw';
 import font from './icons/font.svg?raw';
 import game from './icons/game.svg?raw';
 import group from './icons/group.svg?raw';
@@ -36,6 +37,7 @@ export const ASSET_ICON_SVG: Record<AssetType, string> = {
   camera,
   collider,
   container,
+  curve,
   font,
   palette,
   particles,

src/editor/panels/icons/curve.svg

@@ -0,0 +1,6 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="-8 -8 16 16" width="16" height="16">
+  <!-- An ease-in-out rising from the bottom left to the top right, over the two axes it is drawn on. -->
+  <path d="M-6.5 -7 L-6.5 6.5 L7 6.5" fill="none" stroke="currentColor" stroke-width="1" />
+  <path d="M-6.5 6.5 C0 6.5 0 -6.5 6.5 -6.5" fill="none" stroke="currentColor" stroke-width="1.75" />
+  <circle cx="0" cy="0" r="1.75" fill="currentColor" />
+</svg>