Give the e2e steps one place to build their fixtures

Steps 18 and 20 both hand-rolled the same object-plus-behavior scaffold, down
to lane offsets that nothing kept distinct. Step 19 claimed in a comment that it
cleaned up after itself but never checked it – the only one of the four that
did not.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tools/e2e/lib/scaffold.mjs

@@ -0,0 +1,102 @@
+/**
+ * Steps 17–20 each need the same throwaway rig in the fixture scene: a behavior asset cloned off
+ * `mover` carrying the step's own graph, a crate to hold it, and a logic child that runs it.
+ * Spelled out per step it drifted four ways – `push` vs `unshift`, three different `make`
+ * signatures, and y-offsets nothing kept apart.
+ */
+
+/**
+ * Where each step parks its box. The fixture scene is shared, so two steps at the same y would
+ * overlap and each would fail for reasons belonging to the other.
+ */
+export const LANES = {
+  editorRun: 400,
+  editorRunContainer: 800,
+  editorRunEdits: 1200,
+  pause: 300,
+  nested: 700,
+  keyRepeat: -300,
+};
+
+/**
+ * Source for a `make(id, assetId, parentId, y, name)` factory, to paste inside a `mutate()`
+ * string. It clones `scene.objects[0]`, so a new object carries whatever shape the fixture has.
+ */
+export const MAKE_OBJECT = `
+    const make = (id, assetId, parentId, y, name = '') => {
+      const o = structuredClone(scene.objects[0]);
+      Object.assign(o, {
+        id,
+        assetId,
+        name,
+        position: { x: 0, y, z: 0 },
+        parentId,
+        overrides: {},
+        instanceEdits: {},
+      });
+      return o;
+    };`;
+
+/**
+ * Builds the rig and hands back the ids it created, plus `remove()` which takes them out again
+ * and returns what is left – so a step's cleanup cannot fall out of step with what it made.
+ *
+ * `front` puts the asset at the head of the project list: the asset panel is long by this point
+ * in the suite, and a row appended at the end scrolls out of reach of `dragAsset`.
+ */
+export async function scaffoldBehavior(t, options) {
+  const {
+    assetId,
+    name,
+    graph,
+    boxId,
+    boxName = 'Box',
+    logicId,
+    y,
+    front = false,
+    props = { enabled: true, runInEditor: false },
+  } = options;
+
+  await t.mutate(`
+    const p = __gm.state.project;
+    const scene = __gm.state.activeScene;
+    const asset = structuredClone(p.assets.find((a) => a.id === 'mover'));
+    Object.assign(asset, {
+      id: ${JSON.stringify(assetId)},
+      name: ${JSON.stringify(name)},
+      props: ${JSON.stringify(props)},
+      variables: [],
+      graph: ${JSON.stringify(graph)},
+    });
+    p.assets.${front ? 'unshift' : 'push'}(asset);
+    ${MAKE_OBJECT}
+    scene.objects.push(
+      make(${JSON.stringify(boxId)}, 'crate', '', ${y}, ${JSON.stringify(boxName)}),
+      make(${JSON.stringify(logicId)}, ${JSON.stringify(assetId)}, ${JSON.stringify(boxId)}, 0, ${JSON.stringify(name)}),
+    );
+  `);
+  await t.sleep(400);
+
+  return {
+    assetId,
+    boxId,
+    logicId,
+    /** Removes everything this rig added and reports what survived (should be all zeroes). */
+    async remove() {
+      await t.evaluate('__gm.state.select(null)');
+      await t.mutate(`
+        const p = __gm.state.project;
+        const scene = __gm.state.activeScene;
+        const objects = ${JSON.stringify([boxId, logicId])};
+        const assets = ${JSON.stringify([assetId])};
+        scene.objects = scene.objects.filter((o) => !objects.includes(o.id));
+        p.assets = p.assets.filter((a) => !assets.includes(a.id));
+      `);
+      await t.sleep(300);
+      return t.json(`{
+        objects: __gm.sceneObjects().filter((o) => ${JSON.stringify([boxId, logicId])}.includes(o.id)).length,
+        assets: __gm.state.project.assets.filter((a) => a.id === ${JSON.stringify(assetId)}).length,
+      }`);
+    },
+  };
+}