fix: lazy inotify directory watches for fast startup (closes #324)

[s2x]

Description

Closes #324

Changes

• InotifyMonitorWatcher::start() now watches only top-level source directories at boot instead of doing a synchronous full recursive walk
• InotifyMonitorWatcher::deferredWalk() is a new private method that recursively walks and watches all existing subdirectories, scheduled via Worker::$globalEvent->delay() after the event loop starts
• Added watchedPaths reverse-lookup map for O(1) duplicate detection
• pathByWd cleanup on IN_IGNORED now also removes the path from watchedPaths
• Tests updated: testStartWatchesOnlyTopLevelDirectories (was testStartRecursivelyWatchesDirectories), added testDeferredWalkWatchesAllSubdirectories, and updated event-loop mocks

Changelog

Performance: Optimize InotifyMonitorWatcher startup by deferring the recursive directory walk to after the event loop starts, eliminating synchronous full-directory walk on very large source trees.

Code Review

• Passed subagent code review
• All review comments addressed