fix: allocate stack space for struct undef values in Cranelift JIT

Previously, HirValueKind::Undef for struct types was mapped to iconst(0),
causing InsertValue operations to attempt stores to address 0, resulting
in segfaults during JIT execution.

Now struct undef values are properly allocated on the stack:
- Calculate struct size from field types
- Create stack slot with cranelift_codegen::ir::StackSlotData
- Return stack address pointer instead of zero constant

This enables struct initialization with field access to work correctly:
  const p = Point{ .x = 10, .y = 20 };
  return p.x;  // Returns 10

Author: darmie

crates/compiler/src/cranelift_backend.rs

@@ -702,22 +702,48 @@ impl CraneliftBackend {
             }
 
             // Map undef values to zero constants (for IDF-based SSA)
+            // For struct types, allocate stack space instead of using a zero constant
             for value in function.values.values() {
                 if let HirValueKind::Undef = value.kind {
-                    let ty = type_cache.get(&value.ty).copied().unwrap_or(types::I64);
-                    let cranelift_val = if ty.is_int() {
-                        builder.ins().iconst(ty, 0)
-                    } else if ty.is_float() {
-                        if ty == types::F32 {
-                            builder.ins().f32const(0.0)
-                        } else {
-                            builder.ins().f64const(0.0)
-                        }
+                    // Check if this is a struct type that needs stack allocation
+                    if let HirType::Struct(struct_ty) = &value.ty {
+                        // Allocate stack space for the struct
+                        let struct_size = struct_ty.fields.iter()
+                            .map(|f| match f {
+                                HirType::I8 | HirType::U8 | HirType::Bool => 1,
+                                HirType::I16 | HirType::U16 => 2,
+                                HirType::I32 | HirType::U32 | HirType::F32 => 4,
+                                HirType::I64 | HirType::U64 | HirType::F64 | HirType::Ptr(_) => 8,
+                                HirType::I128 | HirType::U128 => 16,
+                                _ => 8, // Default
+                            })
+                            .sum::<usize>();
+                        let struct_size = std::cmp::max(struct_size, 8) as u32; // At least 8 bytes
+
+                        let slot = builder.create_sized_stack_slot(cranelift_codegen::ir::StackSlotData::new(
+                            cranelift_codegen::ir::StackSlotKind::ExplicitSlot,
+                            struct_size,
+                        ));
+                        let ptr = builder.ins().stack_addr(self.module.target_config().pointer_type(), slot, 0);
+                        self.value_map.insert(value.id, ptr);
+                        eprintln!("[CRANELIFT UNDEF] Allocated {} bytes stack for struct undef {:?}", struct_size, value.id);
                     } else {
-                        // For other types, use null constant
-                        builder.ins().iconst(ty, 0)
-                    };
-                    self.value_map.insert(value.id, cranelift_val);
+                        // For scalar types, use zero constant
+                        let ty = type_cache.get(&value.ty).copied().unwrap_or(types::I64);
+                        let cranelift_val = if ty.is_int() {
+                            builder.ins().iconst(ty, 0)
+                        } else if ty.is_float() {
+                            if ty == types::F32 {
+                                builder.ins().f32const(0.0)
+                            } else {
+                                builder.ins().f64const(0.0)
+                            }
+                        } else {
+                            // For other types, use null constant
+                            builder.ins().iconst(ty, 0)
+                        };
+                        self.value_map.insert(value.id, cranelift_val);
+                    }
                 }
             }