Split search trace service modules

[yvette-carlisle]

Summary

• split packages/elf-service/src/search/trace.rs into a private module root plus endpoint-focused child modules
• move search_explain, trace_get, trace_trajectory_get, trace_recent_list, and trace_bundle_get into normal Rust modules under search/trace/
• preserve existing ElfService method signatures, SQL, validation, and response construction

Validation

• cargo make fmt-rust
• cargo make check-rust
• cargo make lint-vstyle
• cargo make lint-rust
• cargo make test-rust
• cargo make check
• read-only skeptic review: no blockers