fix: move proto code generation output to OUT_DIR, out of tree

[drahnr]

Motivation

Committing generated protobuf code is discouraged and creates review churn. Moving artifacts to OUT_DIR removes ~10k LOC from the repo and avoids frequent diffs when protobuf definitions change. rust-analyzer resolves include! paths, so location is no longer a blocker when it comes to symbol resolution / editor support.

Implementation

We keep the existing integration wiring by continuing to use i.e. generated/mod.rs and include!(concat!(env!("OUT_DIR"), "/generated/…")). This minimizes import-path changes across the codebase and keeps the diff concise.

Side effects

• Removes the docs.rs-specific BUILD_PROTO=1 special casing.
• Proto code is no longer tracked in git -> requires a working protobuf compiler installed

Risks

• Build reproducibility: if protobuf compiler has a breaking change we depend on the host installed compiler

[Mirko-von-Leipzig]

I think is better and minimizes the review diffs when proto changes are involved.

It also makes sense to me to drop the generated/mod.rs codegen but as you say that probably makes more sense as a separate PR. This file can be created manually.

This looks correct; but PR is still in draft and description looks semi-done so I'm not approving yet.

[drahnr]

Added CI and Makefile changes, which were the last missing pieces.

[Mirko-von-Leipzig]

I would hold off on merging this until @bobbinth agrees with this in principle