~anthropic/claude-sonnet-latest → B (3:1)

Side A is a large planning markdown file plus a thin unused RouteContext wrapper that just delegates to ThreadNav—no behavioral change and largely speculative documentation for a future refactor. Side B fixes real clippy warnings, wires up a previously-dead test (missing #[test] attribute meant a test never ran), and makes small correctness/idiomatic improvements (is_some_and, irrefutable pattern cleanup, avoiding unnecessary allocations) across the codebase, providing concrete, lasting code-quality value.