E4-v1 produced 272 admission rejects (good) but zero /_snapshot HTTP
calls (bad, entrance gate bug fixed in e729d62). E4-v2 went the other
way: 0 rejects through 53% of trace, sync function never even called.
E4-pressured locks in the *fix-verified* code path by lowering
--kvcache-migration-reject-threshold from 3 to 1. After ONE
rejection the policy forces session migration, which lands in
_invoke_kvcache_seeded_router → _attempt_d_to_p_sync.
With the e729d62 fix in place, the d-to-p-sync.jsonl structural log
should now capture every prepare/dump/finalize decision so we can
forensic verify the D→P fast path is actually delivering KV bytes
to P's radix tree.
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