feat: pcp-messages protobuf

[karelispanagiotis]

Introduces protobuf definitions for pcp messages to be included in the pcp-package.

[github-actions]

No concrete merge-blocking issues found in the PR diff.

I checked the workspace wiring, proto paths, build.rs generation setup, exported Rust module, and git diff --check passed. I could not run cargo check -p orb-pcp-messages or buf lint in this sandbox because the environment is read-only and lacks buf/protoc, so CI should cover those checks.

[github-actions]

No concrete correctness, security, or regression issues found in the PR diff.

I could not fully verify locally: cargo check was blocked by the read-only sandbox trying to write Cargo/rustup cache state, and buf is not installed in this environment.

[github-actions]

No concrete merge-blocking issues found in the PR diff.

I could not run cargo check -p orb-pcp-messages or buf lint in this sandbox because rustup cannot write to its home directory here, and protoc/buf are not installed. Static review only.

[github-actions]

I found no concrete merge-blocking issues in the PR diff.

I could not run full verification in this environment: cargo check hit a read-only rustup temp path, and buf/protoc are not installed outside the Nix shell here.

[github-actions]

No concrete merge-blocking issues found in the PR diff.

I couldn’t execute the new crate tests here because the read-only environment prevented rustup from creating temp files under /home/runner/.rustup; buf also isn’t installed in this runner. The schema/build wiring looks consistent with the workspace and CI’s nix develop setup.

[github-actions]

I found no concrete merge-blocking issues in the changes introduced by this PR.

Note: I couldn’t run the Rust checks locally because this sandbox is read-only/offline for the required toolchain/dependencies, so this is a static review of the diff.

[github-actions]

No concrete merge-blocking issues found in the PR diff.

I reviewed the new proto definitions, Rust prost crate wiring, workspace inclusion, and Proto CI workflow. I wasn’t able to run the Rust/buf checks locally in this read-only environment, so this is a static review.