Add rust-shell slim devShell + reusable workflows

[thedavidmeister]

Split out from #141 — rust-shell half.

Add the rust-shell slim devShell and the matching reusable workflows. Same shape and motivation as the sol-shell work in #130 / #134 — rust-only consumers (rain.cli, rain.metadoc, anywhere we ship a pure rust binary) shouldn't pull the full default-shell closure (rust + node + chromium + sol + …) just to run their CI.

Status

• rust-shell devShell in flake.nix — `rust-build-inputs ++ rs-tasks ++ common-shell-inputs`
• `rainix-rs.yaml` composite reusable (static + test + wasm)
• `rainix-rs-static.yaml`, `rainix-rs-test.yaml`, `rainix-rs-wasm.yaml` invoke `nix develop github:rainlanguage/rainix#rust-shell`
• Closure regression test (mirror of `sol-shell/closure.test.bats`) — landed in test: add rust-shell closure regression test #160
• `rainix-rs-artifacts` reusable, if we want a separate cargo build job alongside test (currently `cargo test` builds release artifacts implicitly via target-dir caching)

Closes the rust half of #141 once #160 lands.

[thedavidmeister]

Closed by #160 (rust-shell closure regression test) and #163 (dropping the stray rainix-rs-artifacts doc reference).