perf: components by a union-find over the edges

[petercorke]

The labelling of components (nc, component, samecomponent, Vertex.label) was O(V + E) but slow in practice: profiling at 10,000 vertices put 55% of the time in Edge.next, called once per incident edge from adjacent() (and again from predecessors() for a directed graph), each reading two properties.

It is now one pass over the edges with a union-find (path halving), reading the two ends of each edge directly:

• direction is irrelevant by construction, so there is no directed special case (components are still weak);
• the root of a component is its first vertex, so numbering the roots in vertex order gives exactly the same labels as before (checked against the old implementation and against a traversal written out in the tests);
• each vertex's _label is written directly, not through Vertex.__setattr__.

	before	after
10,000 vertices, 30,000 edges	8.7 ms	~4 ms
100,000 vertices, 200,000 edges	136 ms	~40 ms

The "not recomputed when unchanged" test spied on Vertex.adjacent, which this does not call, so it now poisons the labels and checks that they survive until the graph changes. New tests: numbering follows the first vertex whatever the order of the edges, loops and parallel edges, removals and additions are changes, and agreement with an independent traversal on 30 random graphs of both kinds.

755 tests pass, mypy clean.

🤖 Generated with Claude Code

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics 30 complexity · 2 duplication

Metric	Results
Complexity	30
Duplication	2

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.26%. Comparing base (041033f) to head (eb6974f).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #63      +/-   ##
==========================================
+ Coverage   97.24%   97.26%   +0.01%     
==========================================
  Files           3        3              
  Lines        1560     1570      +10     
==========================================
+ Hits         1517     1527      +10     
  Misses         43       43              

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