perf(shapes): vectorize datashader polygon scaling

[timtreis]

Problem

On the datashader shapes path, polygon/multipolygon scaling used a per-geometry Python loop:

_geometry[is_polygon].apply(lambda g: affinity.scale(g, xfact=scale, yfact=scale))

affinity.scale builds a new geometry per shape — a pure-Python O(n) loop that dominates large polygon renders: measured ~60% of a 100k-polygon datashader render (and ~33 s at 1M is almost entirely this loop).

Fix

_scale_geometries: scale every coordinate about each geometry's bounding-box centre (affinity.scale's default origin) in one vectorized pass via shapely.get_coordinates/set_coordinates.

Byte-identical

Verified main-vs-branch on a 2000-polygon datashader render at scale=0.6 and scale=2.0 → diff_px=0, max|d|=0. Unit test test_scale_geometries_matches_affinity_scale locks the helper against shapely.affinity.scale for asymmetric polygons (bbox-centre ≠ centroid), multipolygons, and holes (tolerance 1e-9).

Impact

• ~12× polygons / ~6.5× multipolygons on the scale step → ~2×+ end-to-end at 100k, growing with n.
• Only fires for scale != 1.0 (default skips it; circles use the fast-path, not this branch).
• Peak memory rises (materializes the coord block); negligible at ≤100k — chunking is a possible follow-up at 1M+.
• The affine transform loop is left as-is: vectorizing it is ~0.9× (shapely coordinate (de)serialization dominates, not the Python callback).

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.32%. Comparing base (55f4970) to head (2e4f0b3).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #738      +/-   ##
==========================================
+ Coverage   79.26%   79.32%   +0.05%     
==========================================
  Files          17       17              
  Lines        4596     4604       +8     
  Branches     1028     1030       +2     
==========================================
+ Hits         3643     3652       +9     
+ Misses        603      602       -1     
  Partials      350      350              

Files with missing lines	Coverage Δ
src/spatialdata_plot/pl/render.py	89.69% <100.00%> (+0.13%)	⬆️

... and 1 file with indirect coverage changes

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