Split progressive search follow-up APIs

[yvette-carlisle]

Summary

• Add progressive_search::followup as a private sibling module.
• Move ElfService::search_session_get, search_timeline, and search_details out of service.rs.
• Keep public ElfService methods and progressive-search type exports unchanged.

Validation

• cargo make fmt-rust
• cargo make check-rust
• cargo make lint-vstyle
• cargo make lint-rust
• cargo make test-rust
• cargo make check
• cargo make test-rust-integration
• cargo make check-trace-gate
• git diff --cached --check

Review

• Read-only skeptic review found no blocking refactor findings.