Split retrieval merge policy helpers

[yvette-carlisle]

Summary

• Split retrieval merge/fusion policy into search/ranking/retrieval/merge.rs.
• Kept retrieval.rs as the retrieval owner/reexport surface plus shared payload, matching, neighbor, snippet, and rank helpers.
• Made source-weight, source-priority, source-kind-order, and rank tie-break helpers private to the merge module.
• Added focused coverage for configured source-priority tie-breaks across fusion and recursive retrieval sources.

Validation

• cargo make fmt-rust
• cargo check -p elf-service --all-targets --all-features
• cargo nextest run -p elf-service --all-features merge_retrieval_candidates (3 passed)
• cargo nextest run -p elf-service --lib --all-features search::tests (29 passed)
• cargo make fmt-rust-check
• git diff --check
• cargo make check-rust
• cargo make lint-vstyle
• cargo make lint-rust
• cargo make test-rust (395 passed, 92 skipped)
• cargo make check (395 passed, 92 skipped)

Review

• Scout selected retrieval merge/fusion as the safest next production-code modularization slice.
• Skeptic passed with no blockers or material risks.