[c++] Pre-neatens for polymorphic domainish accessors

[johnkerl]

Issue and/or context: As tracked on issue #2407 / [sc-51048].

This is split out from #2995 to make 2995 simpler.

Note that the intended Python and R API changes are all agreed on and finalized as described in #2407.

These are higher-line-count mods, which would distract from the more substantive mods on 2995.

Changes:

Notes for Reviewer:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.79%. Comparing base (29be284) to head (fba692e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3011      +/-   ##
==========================================
+ Coverage   89.64%   89.79%   +0.14%     
==========================================
  Files          40       40              
  Lines        4096     4096              
==========================================
+ Hits         3672     3678       +6     
+ Misses        424      418       -6     

Flag	Coverage Δ
python	89.79% <ø> (+0.14%)	⬆️

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

Components	Coverage Δ
python_api	89.79% <ø> (+0.14%)	⬆️
libtiledbsoma	∅ <ø> (∅)