Interface RenderInterpolation

Where a frame's §43 render poses come from, and how far between the last two fixed steps it is (§10, §43).

app.on("render", (time) => {
  renderer.render(scene, views, { poseBuffer: poses, alpha: time.interpolationAlpha });
});
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A backend given this record draws the interpolated pose of every node the buffer tracks — position lerped, rotation slerped — instead of the pose the last fixed step left behind; a backend given undefined draws the resolved world transforms (§7). Both are legitimate: the second is what a still frame, a picking pass, or a headless determinism run wants, because it is exactly the simulation state and nothing else.

Nothing here is written to. §43's rule — "the render transform should not feed back into the physics state" — is the reason the pose buffer is passed as a source per frame rather than as something the renderer owns or updates; a renderer only ever calls computeRenderPose (decision, WP-3.6).

The record may be a per-frame scratch object. Application reuses one (plan D7: the frame loop allocates nothing), so a backend must read poseBuffer and alpha during the render call and never retain the record itself — the same rule the views array and the pooled render items follow.