chore: prefix registry variable names with a single underscore

[d-v-b]

closes #3893

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.98%. Comparing base (bd23984) to head (5890548).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3894   +/-   ##
=======================================
  Coverage   92.98%   92.98%           
=======================================
  Files          87       87           
  Lines       11246    11246           
=======================================
  Hits        10457    10457           
  Misses        789      789           

Files with missing lines	Coverage Δ
src/zarr/registry.py	89.47% <100.00%> (ø)

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[d-v-b]

ci failure (docs) and the coverage issues are spurious.

[maxrjones]

ci failure (docs) and the coverage issues are spurious.

fyi docs failures are addressed by #3887