feat: forward stat function docstrings to stat objects

[leotrs]

Summary

• H.nodes.degree? now shows the actual degree docstring (parameters, description) instead of the generic NodeStat class docstring
• One-line change: self.__doc__ = func.__doc__ in IDStat.__init__
• Works for all stat types: node, edge, directed, custom, and with args

Closes #357 (the last remaining point — discoverability was addressed by #699)

Test plan

• New test test_stat_docstring_forwarding covers node stats, edge stats, args preservation, custom stats, and directed hypergraph stats
• Full test suite passes (396 passed)
• Manually verified in IPython

[leotrs]

As requested by @maximelucas recently

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.62%. Comparing base (50db714) to head (a6c8df3).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #708   +/-   ##
=======================================
  Coverage   93.62%   93.62%           
=======================================
  Files          66       66           
  Lines        5161     5162    +1     
=======================================
+ Hits         4832     4833    +1     
  Misses        329      329           

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[nwlandry]

This is awesome, @leotrs! Good to merge, but this makes me wonder if we should remind users somewhere in the docs (maybe quickstart.ipynb or something?) that using the ? is helpful? But this is a separate issue that we could talk about tomorrow in the team meeting.