Prepared per-layer CP HiCache backups can enqueue their final write ack before the radix node is attached. Chunked prefill exposed this window when a separate catch-up backup made writing_check drain the ack queue and pop an unattached node id.
Keep ready but unattached prepared acks queued until insert either attaches the prepared backup or rollback removes the orphan ack. Also document the reactive host free-room eviction plan separately from this state-machine fix.
Constraint: CP HiCache prepared backup transfer can complete before radix insertion attaches node state
Rejected: Drop unknown ack ids | would orphan a later successful prepared attach and leak write state
Rejected: Chunked-only guard | the invalid assumption is in the generic CP write ack state machine
Confidence: high
Scope-risk: narrow
Directive: Do not drain CP write acks unless every ack id is registered in pending_host_backups or ongoing_write_through
Tested: Remote red-green test_cp_hicache_metadata.py::TestHiRadixCacheCPBackup::test_writing_check_defers_unattached_prepared_ack
Tested: Remote PYTHONPATH=python python -m pytest -q test/registered/unit/mem_cache/test_cp_hicache_metadata.py test/registered/unit/mem_cache/test_cp_hicache_load_back_owner_lanes.py (116 passed)
Tested: python -m py_compile python/sglang/srt/mem_cache/hiradix_cache.py test/registered/unit/mem_cache/test_cp_hicache_metadata.py
Not-tested: Full chunked-prefill ETE replay after this commit
Co-authored-by: OmX <omx@oh-my-codex.dev>
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