Split docs and benchmark tests into focused modules

[yvette-carlisle]

Summary

• Split docs unit tests into focused modules for core behavior, put validation, search validation, and source capture.
• Split competitor strength benchmark tests into live-sweep and strength-profile modules.
• Kept normal Rust module boundaries without include! or new #[path] indirection.

Validation

• RUSTC=/run/current-system/sw/bin/rustc cargo make fmt-rust && RUSTC=/run/current-system/sw/bin/rustc cargo make check