docs: fix bare reassignment examples to match compiler behavior

src/content/docs/getting-started/hello-world.mdx

@@ -52,7 +52,7 @@ purus run hello.purus
 ```purus
 const x be 42
 let y be 3.14
-y be 100
+y add be 1     -- y += 1 (compound assignment)
 ```
 
 ## Functions

src/content/docs/ja/getting-started/hello-world.mdx

@@ -52,7 +52,7 @@ purus run hello.purus
 ```purus
 const x be 42
 let y be 3.14
-y be 100
+y add be 1     -- y += 1（複合代入）
 ```
 
 ## 関数

src/content/docs/ja/reference/control-flow.mdx

@@ -119,15 +119,15 @@ for (const item of list) {
 
 ```purus
 while i lt 10
-  i be i add 1
+  i add be 1
 
 until finished
   do-work[]
 ```
 
 ```js
 while (i < 10) {
-  i = i + 1;
+  i += 1;
 }
 while (!(finished)) {
   do_work();

src/content/docs/ja/reference/functions.mdx

@@ -296,15 +296,15 @@ fn count-up limit
   let i be 0
   while i lt limit
     yield i
-    i be i add 1
+    i add be 1
 ```
 
 ```js
 function* countUp(limit) {
   let i = 0;
   while (i < limit) {
     yield i;
-    i = i + 1;
+    i += 1;
   }
 }
 ```

src/content/docs/ja/reference/operators.mdx

@@ -47,9 +47,12 @@ data.method(arg)
 ```purus
 const x be 42
 let y be 10
-y be 20
 ```
 
+:::caution
+裸の識別子への再代入（`y be 20`）は**コンパイルエラー**です（[構文の概要](/ja/reference/syntax/)参照）。プロパティ・インデックス（`obj.x be 1`、`arr[\0] be 1`）と分割代入（`[a; b] be [b; a]`）は宣言キーワードなしで代入でき、可変の変数の更新には複合代入（`y add be 10`）を使います。
+:::
+
 ## 複合代入
 
 算術演算と代入を組み合わせた複合代入演算子:

src/content/docs/reference/control-flow.mdx

@@ -119,15 +119,15 @@ for (const item of list) {
 
 ```purus
 while i lt 10
-  i be i add 1
+  i add be 1
 
 until finished
   do-work[]
 ```
 
 ```js
 while (i < 10) {
-  i = i + 1;
+  i += 1;
 }
 while (!(finished)) {
   do_work();

src/content/docs/reference/functions.mdx

@@ -296,15 +296,15 @@ fn count-up limit
   let i be 0
   while i lt limit
     yield i
-    i be i add 1
+    i add be 1
 ```
 
 ```js
 function* countUp(limit) {
   let i = 0;
   while (i < limit) {
     yield i;
-    i = i + 1;
+    i += 1;
   }
 }
 ```

src/content/docs/reference/operators.mdx

@@ -47,9 +47,12 @@ data.method(arg)
 ```purus
 const x be 42
 let y be 10
-y be 20
 ```
 
+:::caution
+Reassigning a bare identifier (`y be 20`) is a **compile error** — see [Syntax Overview](/reference/syntax/). Property and index targets (`obj.x be 1`, `arr[\0] be 1`) and destructuring (`[a; b] be [b; a]`) assign without a declaration keyword, and mutable variables are updated with compound assignment (`y add be 10`).
+:::
+
 ## Compound Assignment
 
 Compound assignment operators combine an arithmetic operation with assignment: