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/src/export.rs

@@ -131,10 +131,8 @@ pub fn export_module(
 fn collect_external_calls(ops: &[IrOp], ir_ids: &HashSet<WordId>, host_ids: &mut HashSet<WordId>) {
     for op in ops {
         match op {
-            IrOp::Call(id) | IrOp::TailCall(id) => {
-                if !ir_ids.contains(id) {
-                    host_ids.insert(*id);
-                }
+            IrOp::Call(id) | IrOp::TailCall(id) if !ir_ids.contains(id) => {
+                host_ids.insert(*id);
             }
             IrOp::If {
                 then_body,