[ActivePeter]

Summary

• add the native SSD writeback path with bounded byte/item admission, early SSD queueing, O_DIRECT/io_uring persistence, and multiple SSD root support
• keep Owner local commit and ACK independent from background Master route publication
• add native/Foyer backend configuration, reclaim/delete cleanup, and exact-generation SSD readback handling
• add focused coverage for early queue admission, local ACK ordering, SSD readback, replacement generations, and byte-aware write batching

Validation

• git diff --cached --check
• Focused SSD/Owner suite on the 31408 GPU machine: 9/9 passed
• Tool trace replay: 2000/2000 requests completed with OOM=0 in the recorded r6/r7 runs
• Full CI is expected to run from this PR