Add VEP cache source metadata mode

[mwiewior]

Summary

• require VEP cache source metadata (bio.vep.cache_source_type) and reject legacy options_json source selectors
• carry explicit cache source mode through annotate_vep, VCF output, transcript filtering, RefSeq hydration, and cache-builder export paths
• add OpenSpec proposal/design/tasks and tests for metadata parsing, rejected selectors, source mismatch, RefSeq/merged CSQ fields, and RefSeq MT handling

Verification

• cargo fmt --check
• openspec validate add-vep-cache-source-mode --strict
• cargo test -p datafusion-bio-function-vep --lib
• cargo test -p datafusion-bio-function-vep --features cache-builder cache_builder
• cargo clippy -p datafusion-bio-function-vep --all-targets --features cache-builder -- -D warnings
• git diff --check