fix: Rust 1.95 clippy — match guards + map_or

Rust 1.95 promoted collapsible_match and map_unwrap_or; CI runs
-D warnings so they break the build. Collapse nested `if`s into
match guards across codegen/optimizer/export, and swap
map().unwrap_or(..) for map_or / is_ok_and.

crates/core/tests/comparison.rs

@@ -26,8 +26,7 @@ fn probe_gforth(candidate: &str) -> bool {
         .arg("-e")
         .arg("bye")
         .output()
-        .map(|o| o.status.success())
-        .unwrap_or(false)
+        .is_ok_and(|o| o.status.success())
 }
 
 fn find_gforth() -> Option<&'static str> {