feat: 复合赋值 += -= *= /= + CHANGELOG 更新

- lexer: 4 个复合赋值 Token (解析时优先于单字符)
- parser: desugar x+=expr → x=x+expr（零 sema/codegen 改动）
- 新增集成测试 09_compound_assign.l (15 12 24 6)
- CHANGELOG 新增 v0.2.0 条目

src/parser/parser.c

@@ -277,6 +277,37 @@ static AstNode* parse_statement(Parser* p, ErrorInfo* error) {
             value, name->line, name->col);
     }
 
+    // 复合赋值: ident += expr  →  ident = ident + expr
+    if (t->kind == TOK_IDENT) {
+        TokenKind next_kind = (t + 1)->kind;
+        if (next_kind >= TOK_PLUS_EQ && next_kind <= TOK_SLASH_EQ) {
+            const Token* name = advance(p);  // 消费标识符
+            TokenKind comp_op = advance(p)->kind;
+
+            BinaryOp binop;
+            switch (comp_op) {
+                case TOK_PLUS_EQ:  binop = OP_ADD; break;
+                case TOK_MINUS_EQ: binop = OP_SUB; break;
+                case TOK_STAR_EQ:  binop = OP_MUL; break;
+                case TOK_SLASH_EQ: binop = OP_DIV; break;
+                default: break;
+            }
+
+            AstNode* rhs = parse_expr(p, error);
+            if (!rhs) return NULL;
+            if (!expect(p, TOK_SEMICOLON, error, "缺少 ';'")) return NULL;
+
+            AstNode* lhs_ident = ast_make_ident(p->arena,
+                arena_strdup_impl(p->arena, name->start, name->length),
+                name->line, name->col);
+            AstNode* bin_expr = ast_make_binary(p->arena, binop, lhs_ident, rhs,
+                name->line, name->col);
+            return ast_make_assign(p->arena,
+                arena_strdup_impl(p->arena, name->start, name->length),
+                bin_expr, name->line, name->col);
+        }
+    }
+
     // 表达式语句
     AstNode* expr = parse_expr(p, error);
     if (!expr) return NULL;