[python/c++] COO to CSX conversion optimization

[bkmartinjr]

Issue and/or context:

Using scipy.sparse.spmatrix for conversion from SOMA Arrow tables to SciPy csr_matrix is single-threaded and requires contiguous COO input arrays. This PR:

• does the conversion from COO -> CSC/CSR without requiring the extra copy to concat chunked arrays (roughly halving the memory used for this operation).
• parallelizes the conversion - reduces wall-clock time (at the cost of extra total CPU time)

All API/modules are for internal use. There is no external API surface change.

Changes:

• C++ package containing low-level operations. Includes vendoring "span" and "parallel_for" from TileDB repo
• Python module and pybind-ings containing "friendlier" API for internal python code
• Wired into soma.io.to_anndata and the blockwise() reader
• C++ compile flags for Python bindings: added -O3 -mavx2 to setup.py
• Modify R/Python GHA workflow to match latest build practices and remove legacy steps

Later work will wire into additional code paths.

Issue: #3345 [sc-59595]

[codecov]

Codecov Report

Attention: Patch coverage is 98.40000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 85.52%. Comparing base (ddc72c1) to head (ba3f69e).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3304      +/-   ##
==========================================
+ Coverage   85.14%   85.52%   +0.37%     
==========================================
  Files          53       54       +1     
  Lines        5568     5686     +118     
==========================================
+ Hits         4741     4863     +122     
+ Misses        827      823       -4     

Flag	Coverage Δ
python	85.52% <98.40%> (+0.37%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
python_api	85.52% <98.40%> (+0.37%)	⬆️
libtiledbsoma	∅ <ø> (∅)

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[johnkerl]

I'm partway through but need to be AFK. Poking the submit button on where I am now, as a checkpoint.

[johnkerl]

I believe this is simply a copy from core of known-good code -- if so I won't even peek at it

[bkmartinjr]

correct - vendored, as noted in the PR description

[johnkerl]

Sorry for my less-than-thorough read of the PR description!

[johnkerl]

Please see #3328 (review) for narrative.

Thanks @bkmartinjr !

And as on #3328 -- cc @nguyenv for more :eyes.

[nguyenv]

LGTM. Some minor suggestions