Make batching opt-in unless we know it is a good default to have enabled

As in the title. Noting this so we don't forget to do it.

For now, automatic batching is possibly risky to have enabled by default for customised rendering and it may be tricky to debug. As such, @cart suggested that perhaps for things where we know automatic batching works well (StandardMaterial + Mesh, ColorMaterial + Mesh2d, etc) it could be enabled by default, but custom things would have to opt-in.

No files, tests, or entry points are named. Start by locating the automatic batching implementation and its default configuration, then review how StandardMaterial + Mesh and ColorMaterial + Mesh2d are identified. Done means those known-safe combinations remain enabled by default while custom rendering requires explicit opt-in, with coverage for both paths.

Written by the indexing model from the issue text.