feat: 数组+索引 [T;N], arr[i] (P1 #6)

- lexer: TOK_LBRACKET, TOK_RBRACKET
- type: TYPE_ARRAY + TypeInfo扩展(element_type/array_size)
- ast: AST_INDEX_EXPR, AST_ARRAY_ASSIGN_STMT
- parser: parse_type_expr()支持[T;N], Pratt加[索引], 数组元素赋值
- sema: 数组类型检查, 索引必须i64, 元素赋值类型匹配
- codegen: type_info_to_llvm(TYPE_ARRAY), GEP+load/store
- 新增集成测试: 18_array.l

测试: 136 通过 (41+15+59+21)

src/ast/ast.h

@@ -26,6 +26,8 @@ typedef enum {
     AST_TYPE_ALIAS,       // type Meters = i64
     AST_ENUM_DECL,        // enum Color { Red, Green, Blue }
     AST_ENUM_VARIANT,     // Color::Red
+    AST_INDEX_EXPR,       // arr[i]
+    AST_ARRAY_ASSIGN_STMT,// arr[i] = expr
 } AstKind;
 
 typedef enum {
@@ -38,7 +40,10 @@ typedef enum {
 // 类型信息（语义分析阶段填充）
 typedef struct {
     TypeKind kind;
-    const char* struct_name;  // TYPE_STRUCT 时的结构体类型名
+    const char* struct_name;       // TYPE_STRUCT / TYPE_ENUM
+    TypeKind    element_type;       // TYPE_ARRAY: 元素类型的 TypeKind
+    const char* element_struct_name; // TYPE_ARRAY: 元素为 struct 时的类型名
+    int64_t     array_size;         // TYPE_ARRAY: 固定大小
 } TypeInfo;
 
 // AST 节点
@@ -64,7 +69,8 @@ struct AstNode {
         struct { struct AstNode** stmts; size_t stmt_count; } block;
         // AST_LET_STMT
         struct { const char* name; TypeKind annot_type; bool has_type_annot; bool is_mut; struct AstNode* init;
-                 const char* struct_type_name; } let_stmt;
+                 const char* struct_type_name;
+                 TypeKind annot_element_type; const char* annot_element_struct_name; int64_t annot_array_size; } let_stmt;
         // AST_ASSIGN_STMT
         struct { const char* name; struct AstNode* value; } assign_stmt;
         // AST_IF_STMT
@@ -98,6 +104,10 @@ struct AstNode {
         struct { const char* name; const char** variants; size_t variant_count; } enum_decl;
         // AST_ENUM_VARIANT
         struct { const char* enum_name; const char* variant_name; int variant_index; } enum_variant;
+        // AST_INDEX_EXPR
+        struct { struct AstNode* array; struct AstNode* index; } index_expr;
+        // AST_ARRAY_ASSIGN_STMT
+        struct { const char* name; struct AstNode* index; struct AstNode* value; } array_assign;
     } as;
 };
 
@@ -111,7 +121,8 @@ AstNode* ast_make_function(void* alloc, const char* name, AstNode** params, size
 AstNode* ast_make_parameter(void* alloc, const char* name, TypeKind type, const char* struct_type_name, SourceLoc loc);
 AstNode* ast_make_block(void* alloc, AstNode** stmts, size_t count, SourceLoc loc);
 AstNode* ast_make_let(void* alloc, const char* name, TypeKind annot_type, bool has_type_annot,
-                      bool is_mut, AstNode* init, const char* struct_type_name, SourceLoc loc);
+                      bool is_mut, AstNode* init, const char* struct_type_name,
+                      TypeKind annot_elem_type, const char* annot_elem_struct, int64_t annot_array_size, SourceLoc loc);
 AstNode* ast_make_assign(void* alloc, const char* name, AstNode* value, SourceLoc loc);
 AstNode* ast_make_if(void* alloc, AstNode* cond, AstNode* then_b, AstNode* else_b, SourceLoc loc);
 AstNode* ast_make_while(void* alloc, AstNode* cond, AstNode* body, SourceLoc loc);
@@ -132,5 +143,7 @@ AstNode* ast_make_type_alias(void* alloc, const char* name, TypeKind aliased,
                               const char* aliased_struct, SourceLoc loc);
 AstNode* ast_make_enum_decl(void* alloc, const char* name, const char** variants, size_t count, SourceLoc loc);
 AstNode* ast_make_enum_variant(void* alloc, const char* enum_name, const char* variant_name, SourceLoc loc);
+AstNode* ast_make_index_expr(void* alloc, AstNode* array, AstNode* index, SourceLoc loc);
+AstNode* ast_make_array_assign(void* alloc, const char* name, AstNode* index, AstNode* value, SourceLoc loc);
 
 #endif