feat: event-driven CXL device hotplug monitoring

[seohui-XCENA]

Background

Currently, CXL/DAX device detection relies on a timer-based polling mechanism (#48).
This works for the boot-race scenario but has limitations:

• Polling interval introduces unnecessary delay
• No support for runtime device hot-add after initial detection
• No support for device hot-remove (only manual SIGHUP rescan)

Proposal

Replace the polling-based rescan with an event-driven approach using udev kernel event monitoring:

• Monitor CXL subsystem for dax_region add/remove events via udev netlink
• React immediately when devices become available
• Handle both hot-add and hot-remove at runtime
• Remove the polling loop once the event monitor is in place

Scope

• New RegionMonitor component (udev-based)
• Adapt PoolManager to handle dynamic pool add/remove
• Add libudev-dev build dependency
• Graceful handling of active allocations on device removal

Reference

• Depends on: fix: auto-rescan DAX devices when pool is empty on startup #48 (boot-race fix should merge first)
• Prior art: https://github.com/xcena-dev/pxl/pull/293