Add an API to reserve a scene

Nobody has claimed this yet.

Vello should provide a `SceneMetrics` type, which could be returned from a Scene; multiple SceneMetrics could be added together.

Scene should have a new `Scene::reserve(metrics)` constructor taking a SceneMetrics value.

This could be useful for consumers who want to stitch a large amount of pre-existing scenes together in one pass. The consumer would scan over the scene metrics, reserve one giant scene, and then run a second pass to add the sub-scenes to it.

```rust
impl Scene {
fn metrics(&self) -> SceneMetrics;

fn reserve(metrics: &SceneMetrics) -> Self;
}

impl SceneMetrics {
fn add(&self, other: &SceneMetrics) -> SceneMetrics;
}
```

Start by reading the existing Scene API and construction paths to understand how scene storage is allocated and populated. Define how SceneMetrics is computed and combined, then verify that Scene::reserve(metrics) can support adding pre-existing scenes in a second pass; add coverage for metrics aggregation and reserved-scene construction.

Written by the indexing model from the issue text.