Use typed lookup stage formatting

[suchapalaver]

Summary

• add Display for CombinedDataLookupStage and format lookup failures through the typed stage
• keep operation_name() for compatibility as the stable RPC operation label
• record the remaining &'static str surfaces as intentional labels: cache backend names, event/network labels, and test-only scan error payloads

Closes #17.

Verification

• cargo fmt
• nix shell nixpkgs#stdenv.cc -c cargo test --all-features retrieval::types::tests::test_combined_lookup_stage_display_matches_rpc_operation_name
• nix shell nixpkgs#stdenv.cc -c cargo test --all-features
• nix shell nixpkgs#stdenv.cc -c cargo clippy --all-targets --all-features -- -D warnings
• nix shell nixpkgs#reuse -c reuse lint