fix: only proxy actual stats through Hypergraph.__getattr__ (#408)

[leotrs]

Summary

Closes #408. Hypergraph.__getattr__ (and DiHypergraph.__getattr__) blindly delegated every attribute lookup to the underlying views and wrapped whatever it found in a function that calls .asdict(). That works for actual stats but produces confusing errors when users hit view methods that happen to share a name with no stat:

>>> H.filterby()
TypeError: IDView.filterby() missing 2 required positional arguments: 'stat' and 'val'

>>> H.memberships()
AttributeError: 'dict' object has no attribute 'asdict'

>>> H.neighbors()
TypeError: IDView.neighbors() missing 1 required positional argument: 'idx'

Fix

Tighten the delegation check from stat is None to not isinstance(stat, IDStat). Built-in stats (registered via _stat_property) and user-defined stats (via @nodestat_func etc.) both return IDStat instances, so they continue to work. Non-stat view methods (filterby, neighbors, memberships, isolates, singletons, empty, members, dimembers, head, tail, ...) now correctly raise AttributeError at lookup time with the existing helpful message.

Same one-line change in both Hypergraph and DiHypergraph.

Safety check: grepped xgi/, tests/, and tutorials/ for H.filterby(, H.neighbors(, H.memberships(, etc. — zero hits. Nothing in the repo relied on the broken proxies.

Test plan

• Two new regression tests (test_getattr_only_proxies_stats for both Hypergraph and DiHypergraph) covering: built-in stats still work, user-defined stats still work, every non-stat view method raises AttributeError, unknown attributes raise AttributeError
• Full test suite passes (420 passed, 6 skipped)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.63%. Comparing base (9e1e9af) to head (e443951).
⚠️ Report is 43 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #736      +/-   ##
==========================================
+ Coverage   93.65%   94.63%   +0.97%     
==========================================
  Files          66       66              
  Lines        5168     5290     +122     
==========================================
+ Hits         4840     5006     +166     
+ Misses        328      284      -44     

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