feat: add hybrid retrieval API

[dcfocus]

Summary

• Add a retrieve API that combines optional text and vector channels over existing context records.
• Fuse candidate ranks with RRF and return ranking diagnostics: score, vector_distance, text_score, and matched_channels.
• Expose retrieval through Python, Rust DTOs, remote client, and /api/v1/contexts/{name}/retrieve, while preserving existing vector-only search.
• Move filter JSON parsing into core so Python and REST retrieval share the same metadata/lifecycle filter semantics.

Refs #72.

Testing

• cargo-fmt --all -- --check
• cargo test --workspace
• cargo clippy --workspace --all-targets -- -D warnings
• ruff format --check --no-cache python
• ruff check --no-cache python
• pyright
• pytest python/tests/test_search.py python/tests/test_persistence.py::test_retrieve_fuses_text_vector_and_filters python/tests/test_persistence.py::test_retrieve_supports_text_only python/tests/test_async.py::test_retrieve
• pytest python/tests -k "not s3"

Notes

• Full pytest python/tests was also attempted; only the existing moto/S3 persistence tests failed locally with Bucket ... not found, while the non-S3 suite passed.
• The bundled .codex/skills/ci-pr-helper/scripts/run_ci_checks.sh currently points to rust/lance-context/Cargo.toml, which does not exist in this repo layout, so I ran the equivalent current-layout checks directly.