ci: Add coverage reporting to CI

[cbueth]

Summary

Integrate Codecov into the CI pipeline for branch-coverage tracking on PRs, and raise baseline coverage to 100%.

Changes

add test.yml on the latest test run pytest with --cov=delaynet --cov-branch --cov-report=xml and upload via codecov/codecov-action@v5.

codecov.yml, new config enforcing:

• patch target 100%: all new/modified lines must be covered on every PR
• project target auto with 2% threshold: overall coverage must not drop meaningfully
• Codecov app should comment on PR with diff coverage summary

Added Codecov badge

Added few missing lines:

• granger.py: LinAlgError fallback (mock np.linalg.solve to raise)
• metrics.py: removed dead if max_betweenness > 0 branch (always True when reached)
• network_reconstruction.py: direct tests for parallel worker functions using real shared memory

[codecov]

Welcome to Codecov 🎉

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

Thanks for integrating Codecov - We've got you covered ☂️

[codspeed-hq]

Merging this PR will degrade performance by 18.83%

⚠️ 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

❌ 2 regressed benchmarks
✅ 26 untouched benchmarks

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	test_detrend_method[z_score-p10]	1.8 ms	2.3 ms	-22.27%
❌	test_ordinal_patterns	11.6 ms	13.7 ms	-15.25%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing chore/add-codecov-ci (34361ea) with main (0a01312)}