feat: 新增异步 trigger()，使用 pending 计数器

[abin-z]

Body:

Summary

• add asynchronous trigger() support
• keep callable owned by the timer worker without std::function
• preserve all manual trigger requests with a pending counter
• support move-only callables under C++11
• serialize scheduled and manual executions

Behavior

• ignored while paused or stopped
• does not reset the scheduled deadline
• stop/restart clears pending requests
• one-shot timers stop after the first manual or scheduled execution
• callback exceptions stop the timer

Tests

• added 10 trigger-related test cases with 28 assertions
• MSVC Release build and full CTest pass
• explicit C++11 build and full CTest pass
• trigger test suite passed 10 consecutive runs