feat(centroids): add return_area + SpatialData persist to get_centroids

Surface the per-instance area that the labels bincount already computes
(and geometric area for shapes), and add a SpatialData dispatch that
writes centroids/area into the element's annotating table, squidpy-style.

- `return_area` on all element overloads: labels return the pixel/voxel
  count (free, already computed); shapes return `pi*r**2` for circles and
  `geometry.area` for polygons; points raise. Carried as a feature column
  on the returned Points element.
- Harmonise the three element overload signatures (they previously
  disagreed on `return_background`).
- New `get_centroids(sdata, element_name, ..., persist_as="adata")`
  dispatch: resolves the element's annotating table and writes
  `obsm["spatial"]` (x, y[, z]) and `obs["area"]` at the element's rows,
  in place. Raises (pointing to `persist_as="Points"`) when no table
  annotates the element. No standalone AnnData is ever created.
- Element-level `persist_as="adata"` raises, directing to the SpatialData
  form (which can resolve the table).
- Fast path: the table-writing path applies the coordinate transform to
  the tiny per-instance centroid array in-memory and short-circuits when
  it is an identity, avoiding the dask `transform()` round-trip.
  `coordinate_system=None` returns intrinsic coordinates.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: timtreis