Introduce gRPC route helpers

[samuel-williams-shopify]

Introduce Protocol::GRPC::Route as the namespace for parsing and building gRPC request paths.

• Route.parse(path) returns the service and method names.
• Route.build(service_name, method_name) returns the canonical request path.
• Service and method names must follow protobuf identifier syntax; components are not URL-encoded or decoded.
• Query, fragment, path, and percent-encoding delimiters are rejected as invalid gRPC routes.
• Interface#path uses Route.build.
• Deprecated Methods.build_path and Methods.parse_path remain as compatibility wrappers.
• Documentation and server routing examples use the new API.

This lets async-grpc route requests without constructing a partially initialized Call.

Validation:

• bundle exec bake test (232 tests, 420 assertions)
• bundle exec rubocop (58 files, no offenses)
• bundle exec bake decode:index:coverage lib (104/104 public definitions documented)