new: flag rust-toolchain.toml pins in pkh new

src/new/templates/mod.rs

@@ -81,6 +81,11 @@ pub struct ProbeResult {
     /// Installed command / binary name (e.g. the first `[[bin]]` target or
     /// console script).
     pub command: Option<String>,
+    /// Rust toolchain channel pinned by the project's `rust-toolchain.toml`
+    /// (or legacy `rust-toolchain`), e.g. `1.98.0`. Only set by the rust
+    /// template; the chroot build uses the distribution's rustc and ignores
+    /// the pin, so the wizard surfaces it as a heads-up instead.
+    pub toolchain_pin: Option<String>,
 }
 
 /// A package template: one supported ecosystem / build system.