[WIP] [python] Append-mode sketching

[johnkerl]

This is a draft PR, with no reviewers, explicitly marked as not ready for review.

Please do not review this PR.

It is posted here solely for bus-factor risk-reduction, temporarily cloud-hosting some incomplete prototyping work in support of an upcoming second round of design discussion.

Context: #1278

Split-out PRs from this raw material:

• [python] Append-mode pre-check logic #1554
• [python] Append-mode label/ID-mapping logic #1556
• A TBD third PR will do the actual ingest part

[codecov-commenter]

Codecov Report

Patch coverage: 50.12% and project coverage change: +20.92% 🎉

Comparison is base (1d26a56) 64.66% compared to head (8fb284a) 85.58%.
Report is 1 commits behind head on main.

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Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1498       +/-   ##
===========================================
+ Coverage   64.66%   85.58%   +20.92%     
===========================================
  Files         104       34       -70     
  Lines        8686     2942     -5744     
===========================================
- Hits         5617     2518     -3099     
+ Misses       3069      424     -2645     

Flag	Coverage Δ
python	85.58% <50.12%> (-5.72%)	⬇️
r	?

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

Files Changed	Coverage Δ
...ython/src/tiledbsoma/io/registration/signatures.py	27.00% <27.00%> (ø)
...ledbsoma/io/registration/ambient_label_mappings.py	57.22% <57.22%> (ø)
apis/python/src/tiledbsoma/io/ingest.py	82.40% <62.06%> (-2.56%)	⬇️
...thon/src/tiledbsoma/io/registration/id_mappings.py	82.35% <82.35%> (ø)
.../python/src/tiledbsoma/io/registration/__init__.py	100.00% <100.00%> (ø)

... and 74 files with indirect coverage changes

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[johnkerl]

Replaced by #1554 #1556 #1587