Split search ranking payload helpers

[yvette-carlisle]

Summary

• Split Qdrant payload parsing helpers from search/ranking/retrieval.rs into search/ranking/retrieval/payload.rs.
• Keep the existing ranking re-export surface unchanged.
• Preserve collect_chunk_candidates helper order and parsing behavior.

Validation

• cargo make fmt-rust
• cargo check -p elf-service --all-targets --all-features
• cargo make fmt-rust-check
• cargo make check-rust
• cargo make lint-vstyle
• git diff --check
• cargo make lint-rust
• cargo make test-rust
• cargo make check
• cargo make test-rust-integration
• cargo make check-trace-gate
• staged skeptic review: pass, no blockers or risks