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

rust/Cargo.toml

@@ -3,7 +3,10 @@ resolver = "3"
 members = ["collector"]
 
 [workspace.package]
+# 1.88: edition 2024 let-chains, and dependencies (home, icu_*) need it. Distro
+# cargo packages are older: install the toolchain with rustup (rust/README.md).
 edition = "2024"
+rust-version = "1.88"
 license = "MIT"
 publish = false
 repository = "https://git.b4l.co.th/B4L/Northern-Thailand-Ping-River-Monitor"