feat: 列表推导式 [for x in arr: expr] — parser+sema+codegen

AST(29): +AST_LIST_COMP, parser 解析 [for var in expr: body]
sema: 创建子作用域注册循环变量, codegen: for 循环绑定+填充结果数组
已知限制: 仅支持 2 元素及以下数组 (大数组 alloca 对齐问题待修)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ast/ast.c

@@ -109,6 +109,14 @@ AstNode* ast_make_defer_stmt(void* alloc, AstNode* body, SourceLoc loc) {
     return n;
 }
 
+AstNode* ast_make_list_comp(void* alloc, const char* var, AstNode* arr, AstNode* map, SourceLoc loc) {
+    NEW(alloc, AST_LIST_COMP);
+    n->as.list_comp.var_name = var;
+    n->as.list_comp.array = arr;
+    n->as.list_comp.map_expr = map;
+    return n;
+}
+
 AstNode* ast_make_binary(void* alloc, BinaryOp op, AstNode* left, AstNode* right, SourceLoc loc) {
     NEW(alloc, AST_BINARY_EXPR);
     n->as.binary.op = op; n->as.binary.left = left; n->as.binary.right = right;