kernel-cases: SOLIDCRAFT_KERNEL=truck grades as the default kernel

The kernel accepts `truck` (as well as unset) for its truck backend, and
`boolean_kernel()` reports that backend as `default`. The rig compared
the raw setting with that name, so a run with SOLIDCRAFT_KERNEL=truck
reported every case as "this build runs booleans on the default kernel,
not truck" instead of grading it. The rig now names unset, empty and
`truck` alike as `default`.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017itGh5fpMEousnZ91cYcVt

apps/solidcraft-cli/src/kernel_cases.rs

@@ -11,7 +11,16 @@ use crate::{read_json, recipe};
 
 /// The kernel this process runs on (`SOLIDCRAFT_KERNEL`, set by the caller).
 fn kernel() -> String {
-    std::env::var("SOLIDCRAFT_KERNEL").ok().filter(|k| !k.is_empty()).unwrap_or_else(|| "default".into())
+    kernel_name(std::env::var("SOLIDCRAFT_KERNEL").ok().as_deref())
+}
+
+/// The rig's name for a `SOLIDCRAFT_KERNEL` setting: unset, empty or `truck` (all of which the
+/// kernel runs as truck) is `default`, the name `boolean_kernel` reports for truck.
+fn kernel_name(setting: Option<&str>) -> String {
+    match setting.map(str::trim) {
+        None | Some("") | Some("truck") => "default".into(),
+        Some(other) => other.into(),
+    }
 }
 
 /// How many cases a chain of `base` recipes may go through (a ladder case builds on the one
@@ -371,6 +380,15 @@ mod tests {
         let _ = std::fs::remove_dir_all(&root);
     }
 
+    #[test]
+    fn truck_named_explicitly_is_the_default_kernel() {
+        // `SOLIDCRAFT_KERNEL=truck` runs truck, so the rig must not call it a build mismatch.
+        for setting in [None, Some(""), Some(" "), Some("truck")] {
+            assert_eq!(kernel_name(setting), "default");
+        }
+        assert_eq!(kernel_name(Some("smlib")), "smlib");
+    }
+
     #[test]
     fn a_base_cycle_is_an_error_not_a_hang() {
         let root = std::env::temp_dir().join(format!("kernel-cases-cycle-{}", std::process::id()));