feat: match 表达式 (P1 #8 收官)

- lexer: TOK_MATCH, TOK_MATCH_ARROW, TOK_UNDERSCORE
- parser: parse_match_stmt() desugar → let+if-else链
- 零 sema/codegen 改动
- 4个集成测试: enum/int literal/wildcard match

P1 全部完成: type alias + enum + array + impl + match

test/programs/21_match_red.l

@@ -0,0 +1,11 @@
+enum Color { Red, Green, Blue }
+
+fn main() -> i64 {
+    let c = Color::Red;
+    match c {
+        Color::Red => { print_i64(100); }
+        Color::Green => { print_i64(200); }
+        _ => { print_i64(0); }
+    }
+    return 0;
+}