Add type annotations to `snp_data.py` (AnophelesSnpData)

[Yashsingh045]

Problem

malariagen_data/anoph/snp_data.py has an explicit TODO at line 70:
# TODO review type annotations here, maybe can tighten

The file is 2000+ lines and many public methods lack precise return type annotations and parameter types, which limits IDE support and static analysis.

Proposed Fix

• Add return type annotations to public methods (snp_calls, snp_allele_counts, snp_variants, etc.).
• Tighten parameter types using base_params type aliases where applicable.
• Review and tighten internal cache type annotations as suggested in the TODO.

Impact

Better developer experience, improved IDE autocomplete, and catching potential bugs via static analysis.

[Yashsingh045]

Hey @jonbrenas ,
I have opened a PR #1222 for this issue.
Could you please review it when you have time?
Thanks