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/lexer/token.c

@@ -17,6 +17,7 @@ static const char* NAMES[] = {
     [TOK_LT] = "<", [TOK_GT] = ">", [TOK_LT_EQ] = "<=", [TOK_GT_EQ] = ">=",
     [TOK_AND_AND] = "&&", [TOK_PIPE_PIPE] = "||", [TOK_BANG] = "!",
     [TOK_ARROW] = "->",
+    [TOK_PLUS_EQ] = "+=", [TOK_MINUS_EQ] = "-=", [TOK_STAR_EQ] = "*=", [TOK_SLASH_EQ] = "/=",
     [TOK_LPAREN] = "(", [TOK_RPAREN] = ")",
     [TOK_LBRACE] = "{", [TOK_RBRACE] = "}",
     [TOK_COMMA] = ",", [TOK_COLON] = ":", [TOK_SEMICOLON] = ";",