Function compose_overview 

pub fn compose_overview(
    engine: &mut Engine,
    args: OverviewArgs<'_>,
    surface: Surface,
) -> Result<OverviewOutput, ComposeOverviewError>

Compose the overview markdown for either surface.

The function mutates the engine in two well-bounded ways:

1. Calls engine.invalidate_communities() when args.rebuild is true, so the next engine.communities() triggers a fresh Louvain run.
2. engine.communities() itself caches lazily — first call after a write or after invalidation re-computes.

All other accesses are read-only. The function does NOT collect drift warnings or apply chunking — both are the surface’s job.