Extract plot_haplotype_network into AnophelesHapNetAnalysis mixin

[AswaniSahoo]

Relates to #366

Extracts plot_haplotype_network from AnophelesDataResource into a new
AnophelesHapNetAnalysis mixin, as outlined in my comment on the issue.

What this does

• New: malariagen_data/anoph/hapnet.py — mixin with plot_haplotype_network and its internal helpers (_median_joining_network, _mjn_graph)
• Modified: malariagen_data/anopheles.py — removed method, added mixin to MRO, cleaned up unused imports
• Unchanged: hapnet_params.py — kept as-is
• New: tests/anoph/test_hapnet.py — simulated-data tests following the existing hapclust.py pattern

Verification

• MRO resolves correctly with method coming from the new mixin
• All existing test_hapclust tests pass (no regressions)
• ruff check and ruff format clean
• Pre-commit hooks pass

[AswaniSahoo]

@jonbrenas Ready for review when you get a chance. Let me know if you'd like any changes to the test coverage or the MRO placement.