Function recompute 

pub fn recompute(
    render_state: &ArcSwap<RenderState>,
    static_overlay_quads_cell: &ArcSwap<StaticOverlayQuads>,
) -> WorkerDecision

Pure synchronous recompute. Reads the current published SyntaxRenderState, decides whether to recompute, and updates static_overlay_quads_cell accordingly. Returns the decision taken so tests can assert cache-hit / stale-snapshot HOLD / recompute paths without driving the async loop.