fix(display): use execSync shell pipe instead of fs.writeFileSync to avoid EINVAL

Made-with: Cursor

lib/led.js

@@ -1,6 +1,7 @@
 'use strict';
 
 const fs            = require('fs');
+const { execSync }  = require('child_process');
 const log           = require('./logger');
 
 /**
@@ -110,9 +111,10 @@ class Display {
 
   _write(val) {
     try {
-      fs.writeFileSync(DISPLAY_PATH, val);
+      // 用 shell 管道写入，与 echo "..." | tee ... 行为完全一致
+      execSync(`echo "${val}" | tee ${DISPLAY_PATH} > /dev/null`, { timeout: 3000 });
     } catch (e) {
-      log.warn('display', `写入失败 (${DISPLAY_PATH}): ${e.message}`);
+      log.warn('display', `写入失败: ${e.message}`);
     }
   }
 }