Goal: skip P-side re-prefill on reseed path. Push session KV
snapshot from D back to P after each direct-to-D append; reseed
re-uses P's snapshot to fire only the P→D' transfer (no model.forward
on P).
Decision: Option C — D→P snapshot at append-commit, P-side
PrefillSnapshotStore (side-table, not in radix tree), prefill
bypass when snapshot is fresh. Rejects A (radix multi-producer),
B (D→D' direct, fails for session-not-resident), D (eviction-only).
Lays out 8-commit roadmap, wire protocol, failure modes, and the
E4 experiment plan (KVC + D→P vs naive PD-disagg E1 baseline).
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