Replaced third-party buildkit-client git dependency with
wfe-buildkit-protos generated from official moby/buildkit protos.
Direct ControlClient gRPC calls: SolveRequest with frontend attrs,
exporters, cache options. Daemon-local context paths for builds
(session protocol for remote transfer is TODO).
Both proto crates now use tonic 0.14 / prost 0.14 — no transitive
dependency conflicts. 95 combined tests, 85.6% region coverage.
New crate generating Rust gRPC stubs from the official BuildKit
proto files (git submodule from moby/buildkit). Control service,
LLB definitions, session protocols, and source policy.
tonic 0.14 / prost 0.14.
