Split search ranking scope boost helpers

[yvette-carlisle]

Summary

• Split scope-context boost logic from search/ranking/text.rs into search/ranking/text/scope.rs.
• Kept the existing ranking facade for build_scope_context_boost_by_scope.
• Narrowed scope_description_boost to the new module and moved its tests alongside it.

Validation

• cargo make fmt-rust
• cargo make fmt-rust-check
• cargo make check-rust
• cargo make lint-vstyle
• git diff --check
• cargo nextest run -p elf-service search --all-features (56 passed, 150 skipped)
• cargo make lint-rust
• cargo make test-rust (378 passed, 92 skipped)
• cargo make check (378 passed, 92 skipped)
• Docker-backed cargo make test-rust-integration (92 passed, 378 skipped)
• Docker-backed cargo make check-trace-gate

Review

• Scout selected this as the strongest low-risk next modularization slice.
• Skeptic passed after auditing the staged diff, facade path, private helper boundary, behavior preservation, and validation evidence.