build(rust): declare rust-version 1.88; rustup setup for the server

rust/README.md

@@ -72,12 +72,21 @@ Linux needs `pkg-config` and `libssl-dev`.
 
 ### Deploy
 
-On the server, after `git pull` (the Python side of the switch ships in the
-same commit):
+Needs Rust >= 1.88. Distro `cargo` packages are older (`resolver 3 is not
+valid` / `edition 2024` errors), so install the toolchain once with rustup:
 
 ```bash
-cargo build --release --locked --manifest-path rust/Cargo.toml   # as a normal user,
-                                                                  # or BIN=<CI artifact>
+apt-get install -y build-essential pkg-config libssl-dev curl
+curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --profile minimal
+. "$HOME/.cargo/env"        # puts ~/.cargo/bin before /usr/bin/cargo
+cargo --version             # >= 1.88
+```
+
+Then on the server, after `git pull` (the Python side of the switch ships in
+the same commit):
+
+```bash
+cargo build --release --locked --manifest-path rust/Cargo.toml   # or BIN=<CI artifact>
 sudo bash scripts/install_collector.sh             # install + dry run, nothing switched
 sudo bash scripts/install_collector.sh --cutover   # Rust collects from now on
 sudo bash scripts/install_collector.sh --rollback  # back to Python collection