[WIP] Multiscale GraphArrayView

[TeunHuijben]

Add spatial downscaling to GraphArrayView

Rendering large 3D+t graphs in napari materialized full-resolution
volumes: NDChunkCache allocates one whole-volume buffer per timepoint
(~16 GB/frame at 500x2000x2000 int64), and _fill_array painted every
mask at full resolution.

Add a downscale parameter that reduces both. Painting samples the
mask on a strided view anchored to the global output grid, so cost
scales with nnz/f^ndim rather than nnz, and neighboring objects always
sample the same phase. Objects thinner than the factor get a single
voxel at their bbox center so they stay visible and selectable.

Measured on 40 objects of 60k voxels in 64x512x512: 5.9x faster and
16x smaller at downscale=(1, 4, 4), 6.2x/32x with dtype=np.uint32.

Also expose downscale/scale properties, forward downscale and
dtype through to_napari_format, and complete its overloads, which
previously omitted three existing parameters.

I was unaware of #175, will check the overlap

[TeunHuijben]

Note: this PR builds upon #340, for practical reasons in downstream applications.

[JoOkuma]

If I recall correctly, @yfukai had something similar, I might already have this implemented somewhere

[TeunHuijben]

Hi @JoOkuma, correct, I found that after implementing 😅, it is #340. However, that one was quite behind on API. This PR uses exactly the same logic, but fully up to date. Sorry for the duplication...