Support FlexKV Layerwise

[Clebrate]

Description

Add FlexKV layerwise transfer support for the RecSys KVcache manager.

This PR wires the RecSys FlexKV backend to FlexKV's layerwise launch path by:

• adding an optional RECSYS_FLEXKV_ENABLE_LAYERWISE switch;
• passing layerwise_transfer and counter_id to FlexKV onboard launches when enabled;
• adding a RecSys-side eventfd sender for FlexKV layerwise synchronization;
• covering the layerwise onboard path in test_flexkv.py.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.