feat: add_edge() accepts an Edge as well as two vertices

[petercorke]

Summary

graph.add_edge() now accepts an Edge that already carries both vertices, mirroring add_vertex(), which takes coordinates or an existing vertex.

class MyEdge(Edge): ...
g.add_edge(MyEdge(v1, v2, data="a road"))   # was: TypeError, missing v2

The existing add_edge(v1, v2, **kwargs) forms are unchanged, so this is purely additive.

Behavior

• add_edge(e) requires e.v1 and e.v2; otherwise ValueError, with a message pointing at add_edge(v1, v2, edge=e) / e.connect(v1, v2).
• add_edge(e, v2) is a TypeError.
• add_edge(e, cost=...) / data=... is a TypeError: connect() would silently ignore them when given an edge.
• Typed with @overload.

One behavior tightening

add_edge() (both forms) now raises ValueError if the vertices belong to a different graph. Previously g.add_edge(x, y) with x, y from graph h silently added the edge to h.

Tests

tests/test_add_edge_edge.py, 21 tests across UGraph and DGraph. mypy: no new errors.

🤖 Generated with Claude Code

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics 20 complexity · 1 duplication

Metric	Results
Complexity	20
Duplication	1

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[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.59%. Comparing base (cf7e537) to head (2bb12c0).

Files with missing lines	Patch %	Lines
src/pgraph/PGraph.py	90.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #27      +/-   ##
==========================================
+ Coverage   87.37%   87.59%   +0.21%     
==========================================
  Files           2        2              
  Lines         800      814      +14     
==========================================
+ Hits          699      713      +14     
  Misses        101      101              

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