refactor: extract AnophelesIhsAnalysis and add simulated-data tests

[ritoban23]

Part of the ongoing Anopheles God-class refactor (#366).

What this PR does

Extracts all iHS (integrated haplotype score) analysis methods from AnophelesDataResource in anopheles.py into a new dedicated mixin class AnophelesIhsAnalysis, following the same pattern already used by AnophelesH12Analysis, AnophelesG123Analysis, etc.

Files changed

• malariagen_data/anoph/ihs.py (new) — AnophelesIhsAnalysis(AnophelesHapData) mixin containing ihs_gwss, _ihs_gwss, plot_ihs_gwss_track, and plot_ihs_gwss
• malariagen_data/anopheles.py — removed the four iHS methods and abstract cache-name property; added AnophelesIhsAnalysis import and inserted it into the MRO
• tests/anoph/test_ihs.py (new) — 7 test functions × 2 simulated-data fixtures (ag3, af1) = 14 parametrised tests covering basic runs, caching, windowing, sample queries, and percentile shapes

Testing

• pytest test_ihs.py # 14 passed
• pytest test_h12.py test_g123.py test_h1x.py # no regressions