gRPC/Protobuf/Twirp caching research #226
Description
Design three-crate architecture:
- hitbox-protobuf — shared protobuf cache key extraction and message-level predicates. Runtime inspection via `prost-reflect` + optional codegen for zero-overhead typed access.
- hitbox-tonic — tower layer for tonic, gRPC predicates (service/method path matching, unary-only filtering, `tonic::Status` handling). Depends on `hitbox-tower` + `hitbox-protobuf`.
- hitbox-twirp — integration with `twirp-rs`, HTTP predicates for `/twirp/` routes, content-type negotiation protobuf/JSON. Depends on `hitbox-http` + `hitbox-protobuf`.
Depends on predicate/extractor API unification (PR #202). Deliverable: design document.