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>

README.md

@@ -43,7 +43,7 @@ npm run build        # writes dist/ and regenerates index.html; the folder is wh
 The browser suite and the three example games need a machine with a GPU:
 
 ```bash
-node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 1459 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 1472 checks (GM_STEPS=20 for one step)
                                                    #   every tool signs in first – see the Accounts page
 node tools/seed-jumprun.mjs && node tools/smoke-jumprun.mjs
 node tools/seed-tetris.mjs && node tools/smoke-tetris.mjs
@@ -72,6 +72,7 @@ CI. Details, the GPU requirement and the detached runner are in
 | [Fonts](https://git.nockal.com/meleki/game-maker/wiki/Fonts) | Font assets, the picker, what a text falls back to |
 | [Sounds](https://git.nockal.com/meleki/game-maker/wiki/Sounds) | Sound assets and `Play sound` |
 | [Animations](https://git.nockal.com/meleki/game-maker/wiki/Animations) | Animation assets and the Animation tab |
+| [Curves](https://git.nockal.com/meleki/game-maker/wiki/Curves) | Easings drawn by hand, the Curve tab, using one on a key or a `Lerp` |
 | [Shaders](https://git.nockal.com/meleki/game-maker/wiki/Shaders) | Shader graphs a sprite wears, their node set, the properties they declare |
 | [Particles](https://git.nockal.com/meleki/game-maker/wiki/Particles) | Particle graphs: a particle as a function of its age |
 | [State machines](https://git.nockal.com/meleki/game-maker/wiki/State-machines) | The asset, named transitions, `State branch`, `Set state` and `State transition` |