Replace U+00B5 (MICRO SIGN) with plain 'us' to avoid Unicode warning

Gitea flags U+00B5 as ambiguous — visually identical to U+03BC
(GREEK SMALL LETTER MU). Using 'us' is the standard engineering
abbreviation and avoids the warning.

shm/RuleStatShm.h

@@ -53,7 +53,7 @@ struct ShmSpinLock {
         continue;
       }
       // 持有者存活，短暂退避
-      struct timespec ts = {0, 100000}; // 100µs
+      struct timespec ts = {0, 100000}; // 100us
       ::nanosleep(&ts, nullptr);
     }
   }