Add nrefs() method to VirtualiZarrDatasetAccessor

[maitriupadhyay03-cell]

What I did

Adds a nrefs() method to VirtualiZarrDatasetAccessor that returns the total number of virtual chunk references in a dataset, ignoring non-virtual (loadable) variables. Useful for knowing how many references you're about to write to Icechunk or Kerchunk before actually doing it.

The implementation follows the same pattern as the existing .nbytes property.

Acceptance criteria:

• Closes Add .nrefs method to accessor that returns number of virtual chunk references #573
• Tests added
• Tests passing
• No test coverage regression
• Full type hint coverage
• Changes are documented in docs/about/releases.md
• New functions/methods are listed in an appropriate *.md file under docs/api
• New functionality has documentation

[TomNicholas]

Great, thank you!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.07%. Comparing base (a1fa95c) to head (cbb718f).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1029      +/-   ##
==========================================
+ Coverage   90.01%   90.07%   +0.05%     
==========================================
  Files          36       36              
  Lines        2204     2206       +2     
==========================================
+ Hits         1984     1987       +3     
+ Misses        220      219       -1     

Files with missing lines	Coverage Δ
virtualizarr/accessor.py	95.74% <100.00%> (+0.09%)	⬆️

... and 1 file with indirect coverage changes

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