Add anthropics/buffa-packaging plugin v0.3.0

[iainmcgin]

Adds the protoc-gen-buffa-packaging plugin for assembling buffa-style per-file output into a single mod.rs module tree.

What this plugin does

protoc-gen-buffa-packaging reads the proto package structure (not message/service bodies) and writes a mod.rs that include!s each generated file at the right module nesting level. It works with any codegen plugin that emits per-file output named via the buffa naming convention (foo/v1/bar.proto -> foo.v1.bar.rs), which includes protoc-gen-buffa itself and plugins layered on top of it (like protoc-gen-connect-rust).

Why register this in BSR

The buffa plugin (#2333) emits per-file output without a mod.rs. For local buf generate users, the packaging plugin assembles the module tree so the generated code can be used as pub mod proto; from a single #[path]. BSR's lib.rs synthesis handles the SDK-crate case differently, but BSR users who pull the plugin into a buf.gen.yaml workflow still need this packaging step.

Options

• filter=services — only include proto files that declare at least one service. Useful when packaging output from a service-stub generator that skips files without services.

The plugin requires strategy: all (client-side concern in buf.gen.yaml) so it sees the full file set in one invocation.

No registry block

This plugin emits only mod.rs files containing #[path] and pub mod declarations - no Rust code with cargo deps. So no registry: block is needed (same pattern as community/neoeinstein-prost-crate).

Dependencies

Logically pairs with #2333 (anthropics/buffa) and #2334 (anthropics/connect-rust). The packaging plugin works on output from those plugins but does not declare them as plugin deps - it operates on the descriptor set, not on the generated code.

Process note

CONTRIBUTING.md asks for an issue first - happy to open one if you would prefer to discuss before reviewing. Filed as draft.

[iainmcgin]

I'll leave this as draft until we determine if it makes sense for this to be exposed as a remote plugin, or if we should treat this as a local-only helper.