feat(io): allow a custom parquet writer for points elements

[cornhundred]

Adds an optional points_writer hook to SpatialData.write() and write_points(), letting a caller control the points.parquet layout (row-group boundaries, compression, number of files) while SpatialData still writes all element metadata. Default behaviour is unchanged when the hook is omitted.

This follows the existing pass-through option pattern on write() (shapes_geometry_encoding, raster_compressor,
convert_table_strings_to_categoricals) rather than introducing a new concept, and it is deliberately layout-generic: nothing about it is specific to any tiling scheme or viewer.

Motivation: producing spatially tiled row groups otherwise requires writing the points parquet once with the default writer and then rewriting it. On an 8M transcript Xenium dataset that measured 13.5s vs 8.5s, and it writes ~250MB of transient output that is immediately discarded -- a gap that widens on 300M-transcript panels.

Note the hook receives the dataframe after transformations are stripped from attrs (as the default writer requires), so a writer needing the coordinate transform must obtain it from the element beforehand.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.89%. Comparing base (ccf1ea0) to head (229f1a8).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1201   +/-   ##
=======================================
  Coverage   91.89%   91.89%           
=======================================
  Files          53       53           
  Lines        7942     7947    +5     
=======================================
+ Hits         7298     7303    +5     
  Misses        644      644           

Files with missing lines	Coverage Δ
src/spatialdata/_core/spatialdata.py	93.86% <ø> (ø)
src/spatialdata/_io/io_points.py	100.00% <100.00%> (ø)

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