fix: for-in var 头撞既有绑定复用槽位消除顶层编译报错，迭代键写不可写全局内置接只读拦截

crates/oxide_emit/src/stmt/iteration/for_in.rs

@@ -38,8 +38,25 @@ impl Emitter {
                             let is_const = matches!(decl.kind, VariableDeclarationKind::Const);
                             let name = bi.name.as_str();
                             let var_reg = ctx.alloc_reg();
-                            ctx.declare(name, var_reg, decl.kind, is_const)?;
-                            if let Some(&cell_idx) = ctx.captured_bindings.get(name) {
+                            // var 声明：绑定已存在（顶层 var 预声明、GDI 序言预登记
+                            // builtin 名、同 scope 先前声明）时复用既有槽位，不重复
+                            // 声明；let/const 同 scope 重复声明仍报错。
+                            let target_reg = if matches!(decl.kind, VariableDeclarationKind::Var) {
+                                match ctx.declare(name, var_reg, decl.kind, is_const) {
+                                    Ok(()) => var_reg,
+                                    Err(_) => ctx.lookup(name).unwrap_or(var_reg),
+                                }
+                            } else {
+                                ctx.declare(name, var_reg, decl.kind, is_const)?;
+                                var_reg
+                            };
+                            if ctx.targets_readonly_builtin(name, target_reg) {
+                                // 声明撞全局不可写内置：声明不更新既有全局绑定——sloppy
+                                // 静默跳过写（槽保留入口预载原值），strict 抛 TypeError。
+                                if ctx.is_strict {
+                                    self.emit_throw_error("TypeError", "cannot assign to read-only property", ctx)?;
+                                }
+                            } else if let Some(&cell_idx) = ctx.captured_bindings.get(name) {
                                 let op = if fresh_cell { OpCode::MAKE_CELL_FRESH } else { OpCode::MAKE_CELL };
                                 ctx.inst(Inst::new(
                                     op,
@@ -50,7 +67,7 @@ impl Emitter {
                             } else {
                                 ctx.inst(Inst::new(
                                     OpCode::STORE_VAR,
-                                    Operand::Reg(var_reg),
+                                    Operand::Reg(target_reg),
                                     Operand::Reg(key_reg),
                                     Operand::None,
                                 ));

crates/oxide_vm/tests/for_in_ordering_tests.rs

@@ -79,3 +79,24 @@ fn for_in_still_enumerates_own_keys() {
         "for-in still enumerates the object's own enumerable keys"
     );
 }
+
+#[test]
+fn for_in_var_head_reuses_existing_var_binding() {
+    // var 头命中同 scope 已预声明的 var 绑定：复用既有槽位不重复声明，
+    // 迭代键逐迭代落入该绑定（终值 = 末键）。
+    assert_eq!(
+        eval("var x = 0; for (var x in {a:1,b:2}) {} x === \"b\""),
+        "true",
+        "for-in var head reuses the predeclared same-scope var binding"
+    );
+}
+
+#[test]
+fn for_in_var_head_fresh_name() {
+    // var 头无其他引用：提升的 var 槽接收迭代键。
+    assert_eq!(
+        eval("for (var x in {a:1}) {} x === \"a\""),
+        "true",
+        "for-in var head binds iteration keys into the hoisted var slot"
+    );
+}

crates/oxide_vm/tests/global_builtin_constant_descriptor_tests.rs

@@ -186,3 +186,21 @@ fn function_declaration_builtin_name() {
          && d.enumerable === false && d.configurable === true",
     );
 }
+
+#[test]
+fn for_in_var_head_builtin_name_compiles_and_keeps_property() {
+    // 顶层 for-in var 头撞 builtin 名：序言预登记槽复用后可编译，
+    // 迭代键不触及全局属性（Math 对象保）。
+    run_truthy("var m = Math; for (var Math in {a:1,b:2}) {} globalThis.Math === m");
+}
+
+#[test]
+fn for_in_var_head_readonly_builtin_not_clobbered() {
+    // 顶层 for-in var 头撞三常量：迭代键写命中只读内置拦截——
+    // sloppy 静默跳过（镜像槽与声明值均不污染），strict 迭代抛 TypeError。
+    run_truthy("var n = NaN; for (var NaN in {a:1,b:2}) {} Number.isNaN(NaN) && Number.isNaN(n)");
+    run_truthy(
+        "var i = Infinity; for (var Infinity in {a:1,b:2}) {} Infinity === i && Infinity === globalThis.Infinity",
+    );
+    run_truthy("\"use strict\"; try { for (var NaN in {a:1}) {} } catch (e) { e instanceof TypeError }");
+}