perf(render_images): stream multi-channel composite (avoid O(n_channels) RGBA cube)

[timtreis]

Closes #734.

Problem

Five multi-channel composite branches in _render_images build the full (n_channels, H, W, 4) float64 cube via np.stack([cmap(layers[ch]) for ch in channels], 0).sum(0) before reducing — peak ≈ 64·n·H·W bytes (GB-scale on large multiplex images).

Fix

A streaming helper _composite_channels accumulates per-channel RGB into a single (H, W, 3) buffer (O(1) full buffers), mirroring the existing n>3 branch.

Byte-identical

numpy.sum(axis=0) over the stacked outer axis reduces sequentially, matching acc += rgb — so the result is bit-for-bit identical to the old stack-and-sum (incl. >128 channels, past numpy's pairwise threshold). Verified end-to-end main-vs-branch across 8 render cases (max|Δ| = 0) and locked by a regression test.

Impact

• Peak memory 3357 MB → 26 MB (129×) at 200 channels @512².
• The n>3 premultiplied-alpha branch and the RGB-direct branch keep their own math and are untouched.

Tests

TestCompositeChannels: asserts the helper equals np.stack(...).sum(0)[:, :, :3] across channel counts (2, 3, 7, 129, 200) and that it returns an owned C-contiguous float64 (H,W,3) buffer.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 79.27%. Comparing base (55f4970) to head (15c6c90).

Files with missing lines	Patch %	Lines
src/spatialdata_plot/pl/render.py	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #736      +/-   ##
==========================================
+ Coverage   79.26%   79.27%   +0.01%     
==========================================
  Files          17       17              
  Lines        4596     4594       -2     
  Branches     1028     1029       +1     
==========================================
- Hits         3643     3642       -1     
+ Misses        603      602       -1     
  Partials      350      350              

Files with missing lines	Coverage Δ
src/spatialdata_plot/pl/render.py	89.63% <90.90%> (+0.08%)	⬆️

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