Fix docs build (detached copytable/addrows! docstrings) + docstring placement lint

[barrettp]

Phase 380 put helpers between the copytable / addrows! docstrings and their methods, dropping the docstrings; the Documentation job fails on main. Helpers moved above; new test/docstring_tests.jl fails if any docstring in src/ or ext/ is separated from its definition (verified it fails on the Phase 380 layout). Local docs/make.jl builds with no errors.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.04%. Comparing base (e874a39) to head (e99a98a).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #122   +/-   ##
=======================================
  Coverage   96.04%   96.04%           
=======================================
  Files          52       52           
  Lines       10632    10632           
=======================================
  Hits        10211    10211           
  Misses        421      421           

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