make bootstrap should fully bootstrap the dev environment (rustup + pinned host tools)

[adam2k]

Follow-up from a review comment on #2032 by @NickeZ:

We could make make bootstrap also install rustup and these tools and get rid of the versions in the Dockerfile. I think long term it would be good to have a make bootstrap that completely bootstraps the dev environment. Very good for short lived AI boxes.

(#2032 (comment))

Problem

The host toolchain versions live only in the Dockerfile:

RUN curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | CARGO_HOME=/opt/cargo sh -s -- --default-toolchain $(grep -oP '(?<=channel = ")[^"]+' rust-toolchain.toml) -y
RUN CARGO_HOME=/opt/cargo cargo install cbindgen --version 0.29.2 --locked
RUN CARGO_HOME=/opt/cargo cargo install bindgen-cli --version 0.72.1 --locked
RUN CARGO_HOME=/opt/cargo cargo install --path prost-build-proto --locked

Anyone building natively (macOS, or a fresh Linux box) has to replicate those by hand. #2032 documents them in BUILD.md, which helps, but now the versions are duplicated in two places and will drift.

Today make bootstrap only does:

bootstrap:
	git submodule update --init --recursive
	./scripts/bootstrap-cargo-config

Proposal

Make make bootstrap fully bootstrap a dev environment:

• Install/verify rustup and the toolchain pinned by src/rust/rust-toolchain.toml.
• Install cbindgen, bindgen-cli, and prost-build-proto at pinned versions.
• Keep the existing submodule + cargo-config steps.
• Single source of truth for the versions (e.g. a small file or variables in the Makefile) consumed by both make bootstrap and the Dockerfile, so the container and native builds cannot drift.
• Be idempotent and skip work when the right versions are already installed.
• Trim the corresponding manual steps from BUILD.md once this lands.

Open questions

• Should make bootstrap install rustup itself (network + shell pipe), or detect it and instruct the user to install it?
• Does this need to work on both macOS and Linux, and how do the brew-installed dependencies (arm-gcc, etc.) fit in?
• Should there be a separate make bootstrap-check for CI to assert no drift?