feat: SE2Metric, a weight on the angle of the SE2 metric

[petercorke]

The built-in "SE2" metric adds radians to metres with equal weight, and there is no way to change that: in a test 144 of 300 nearest-vertex answers differed between a weight of 1 and 0.1.

SE2Metric(weight) is the same norm with the wrapped angle multiplied by a weight, the length that a radian is worth:

d = sqrt(dx² + dy² + (weight · wrap(dθ))²)
g = UGraph(dim=3, metric=SE2Metric(weight=0.5), heuristic=SE2Metric(weight=0))

• "SE2" is SE2Metric() (weight 1), so existing results are unchanged.
• Edge costs, closest, Vertex.distance and the A* heuristic all go through g.metric / g.heuristic, so they use the weight with no other change. A weighted L2 on a circle is still a metric, so it is a consistent heuristic when costs come from the metric; SE2Metric(weight=0) (planar distance) is the safe heuristic when costs come from elsewhere.
• An instance pickles with its weight and compares equal by weight. SE2Metric is exported from pgraph.
• It no longer writes into its argument (the old _SE2 assigned v[2]).

Docs: a "Distance metrics" section in construction.rst with a worked example, the metric/heuristic docstrings, and the intro.

21 new tests, including A* against uniform-cost search at four weights. 625 pass, mypy clean, docs build with no warnings.

This is the first of two PRs; the second (spatial queries, a k-d tree for closest/near) needs the weight.

🤖 Generated with Claude Code

[codacy-production]

Not up to standards ⛔

🔴 Issues 6 critical

Alerts:
⚠ 6 issues (≤ 0 issues of at least minor severity)

Results:
6 new issues

Category	Results
Security	6 critical

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🟢 Metrics 27 complexity · 0 duplication

Metric	Results
Complexity	27
Duplication	0

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.71%. Comparing base (bc8e844) to head (4990b85).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #58      +/-   ##
==========================================
+ Coverage   96.61%   96.71%   +0.10%     
==========================================
  Files           3        3              
  Lines        1358     1371      +13     
==========================================
+ Hits         1312     1326      +14     
+ Misses         46       45       -1     

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