Rewrite Variant Python thin wrapper

[dmartmillan]

Description
Reduce openvariant/variant/variant.py to a thin wrapper over _core.VariantReader. Preserve iterator protocol and where parameter.

Files

• openvariant/variant/variant.py — REWRITE
• openvariant/variant/where.py — REWRITE (shim to Rust parser)

Definition of Done

• Variant(path, annotation, where="CHR == 1") still works
• for row in variant.read() still yields dict[str, str]
• where.py delegates to _core parse, kept only for any external import compat

Documentation needed: No external API changes.

Tests needed:

• Existing tests/test_variant.py must pass without modification
• Existing tests/test_where.py must pass without modification