feat(web): recall shadow canary viewer

[srobinson]

Summary

• Add a read only /api/recall-shadow endpoint with routing, scope path, top 1 change, and clamped limit filters.
• Add ts-rs generated recall shadow row types plus frontend client and React Query hook wiring.
• Add a dashboard panel with divergence, average overlap, total rows, filters, and linked old versus new top K IDs.
• Add backend parity coverage and source-level frontend contracts for loading, empty, and populated panel states.

Test plan

• cargo clippy --workspace --all-targets -- -D warnings
• just test
• just build
• just web-check