Shared code generation logic for buffa.

This crate takes protobuf descriptors (google.protobuf.FileDescriptorProto, decoded from binary FileDescriptorSet data) and emits Rust source code that uses the buffa runtime.

It is used by:

• protoc-gen-buffa (protoc plugin)
• buffa-build (build.rs integration)

Architecture

The code generator is intentionally decoupled from how descriptors are obtained. It receives fully-resolved FileDescriptorProtos and produces Rust source strings. This means:

• It doesn't parse .proto files.
• It doesn't invoke protoc.
• It doesn't do import resolution or name linking.

All of that is handled upstream (by protoc, buf, or a future parser).