Foundation: make Logger::shutdown() channel-detach instead of map-reset (fix UAF)

[SAY-5]

Fixes #5324.

Logger::shutdown() previously did:

void Logger::shutdown()
{
    Mutex::ScopedLock lock(_mapMtx);
    _pLoggerMap.reset();
}

shutdown() is called from LoggingSubsystem::uninitialize(), which runs before the destruction of static singletons that registered via atexit. Those singletons frequently cache a Logger& obtained from Logger::get() during construction — the canonical pattern recommended in the Poco samples and used widely by downstream code. Once the logger map is torn down, every such cached reference is dangling, and the first log call from a static destructor hits a heap-use-after-free.

This switches shutdown() to detach the channel from every Logger rather than destroying them. logImpl() already short-circuits on if (_pChannel), so once the channel is null all further logging from a Logger is a cheap no-op and the reference stays valid. The map itself is released at real program exit when the static _pLoggerMap is destroyed; no long-lived singleton can touch it between shutdown() and that point.

[aleks-f]

#5325