Pipe the solver's stderr instead of inheriting it

CLAUDE.md

@@ -26,3 +26,6 @@ keep from reaching it.
 Neither prerequisite of `cargo test` is set up in the session container. Install Z3 at the
 version `.github/actions/setup-z3` pins for CI, and start a Docker daemon with `dockerd &`.
 The daemon dies from time to time, so restart it whenever the tests that need it fail.
+
+Export the `COAR_IMAGE` digest that `.github/workflows/ci.yml` pins, and `docker pull` it
+before running `cargo test`.

src/chc/solver.rs

@@ -75,6 +75,7 @@ impl CommandConfig {
             .args(&self.args)
             .arg(path_arg)
             .stdout(stdout)
+            .stderr(std::process::Stdio::piped())
             .spawn()?;
         tracing::info!(program = self.name, args = ?self.args, path = %path_arg.to_string_lossy(), pid = child.id(), "spawned");