perf(metrics): vectorize global_efficiency inner loop

[cbueth]

This PR replaces the pure-Python double for-loop over igraph's shortest-path distance matrix with numpy vectorised operations. The original code iterated dist_matrix[i][j] pair by pair, performing one Python division at a time.

The vectorized version converts the list-of-lists to a numpy array, creates a boolean mask for reachable non-diagonal pairs, and computes 1.0 / distance in a single vectorised operation. Now it uses np.sum, not +=.

Also replaces the fragile test_global_efficiency_pair_count_zero test (which mocked builtins.range as an implementation detail of the loop) with test_global_efficiency_all_paths_infinite that mocks only igraph, the actual external dependency.

This change was benchmarked on 50-node networks. The impact is negligible since igraph's shortest-path computation dominates, but the elimination of N(N-1) Python iterations removes a latent bottleneck for larger networks or repeated calls under normalisation.

[codspeed-hq]

Merging this PR will improve performance by 36.75%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 4 improved benchmarks
✅ 24 untouched benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	test_network_metric[global_efficiency-dir]	4.3 ms	3.1 ms	+42.07%
⚡	test_network_metric[global_efficiency-undir]	4.4 ms	3.1 ms	+41.04%
⚡	test_ordinal_patterns	15.9 ms	11.9 ms	+33.61%
⚡	test_detrend_method[z_score-p10]	2.1 ms	1.6 ms	+30.63%

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing perf/vectorize-global-efficiency (592d2fc) with main (875adcd)}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (875adcd) to head (592d2fc).

Additional details and impacted files

@@            Coverage Diff            @@
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  Coverage   100.00%   100.00%           
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  Files           29        29           
  Lines          842       836    -6     
  Branches       151       146    -5     
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- Hits           842       836    -6     

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