fix: deprecate adjacent()/vertices() properly instead of hard-breaking

[petercorke]

Summary

• UVertex.adjacent() had been removed outright with no migration path -- renamed to neighbours()/neighbors() with zero deprecation phase, so any caller still using the old name got a bare AttributeError. Restored it as a thin wrapper: still calls neighbours() and returns its result, but now emits a DeprecationWarning first, giving callers a working upgrade path instead of an immediate break.
• Also fixed the same underlying anti-pattern already present nearby in Edge.vertices(): it did raise DeprecationWarning(...), which aborts the call exactly like a missing method would (DeprecationWarning is just an Exception subclass when raised directly) -- no backward compatibility at all, and it never goes through the actual warnings machinery (no -W flag control, no filterwarnings support). Now uses warnings.warn(..., DeprecationWarning, stacklevel=2) and still returns self.endpoints, matching the same real deprecation pattern as adjacent() above.

Test plan

• `tests/test_graph.py::TestUGraph::test_adjacent_deprecated` (new) -- `adjacent()` still returns the same result as `neighbours()`, wrapped in `assertWarns(DeprecationWarning)`
• `tests/test_graph.py::TestUGraph::test_edge_vertices_deprecated` (new) -- `vertices()` still returns the same result as `endpoints`, wrapped in `assertWarns(DeprecationWarning)`
• Full test suite: 34 passed, no regressions
• Manually verified against RVC3-python's `app.ipynb` notebook (`g[1].adjacent()`), which was hitting the original hard-break

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.28%. Comparing base (13582c4) to head (c29e3f3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #2      +/-   ##
==========================================
+ Coverage   81.88%   82.28%   +0.40%     
==========================================
  Files           2        2              
  Lines         712      717       +5     
==========================================
+ Hits          583      590       +7     
+ Misses        129      127       -2     

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