Serialize folder monitor state

[sickn33]

Summary

• serialize FolderMonitor start/stop state mutations on its private queue
• make isMonitoring read the same queue-owned state
• capture the file descriptor owned by each dispatch source so a rapid stop/start cannot close the new descriptor from an old cancel handler
• add a start/stop state test

Why

FolderMonitor handled file-system events on a private queue, but public start/stop mutated source, watchedURL, knownFiles, and generation from the caller thread. That leaves room for races between delayed filesystem scans and settings changes. The old cancel handler also read self.fileDescriptor, which can point to a newer descriptor if monitoring is restarted before the old source finishes cancelling.

Verification

• git diff --check
• xcodebuild test -project DropKit/DropKit.xcodeproj -scheme DropKit -destination 'platform=macOS' -derivedDataPath /private/tmp/dropkit-foldermonitor-derived CODE_SIGNING_ALLOWED=NO passed 12 tests, 0 failures