fix: single layer ng annotation in collection results in 0 z-size bbox

[teo-zetta]

Using shrink mode from snapped() function reduces bbox z-size to 0. The proper way is to floor the coordinates instead and matches the way neuroglancer handles coordinates.

Background:
Coordinates are stored in units of 1nm. snapped() is normally used to align bboxes to chunks, this was repurposed to align to resolution coordinates. Using shrink would always add 1 to each coordinate when the correct behavior should be just flooring. Thus a new floor mode is added to snapped() and build_collection_dataset() is changed to use the new floor mode.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.98%. Comparing base (13cf657) to head (6f0cf5b).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1203   +/-   ##
=======================================
  Coverage   99.98%   99.98%           
=======================================
  Files         192      192           
  Lines       10203    10206    +3     
=======================================
+ Hits        10201    10204    +3     
  Misses          2        2           

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