DC-EPOCH-20

Atomic-or-rematerialized selected-block admission -- no RESUMED split authority. For every selected block admitted to the durable chain, four derived authorities advance from the SAME selected-chain prefix: the ChainDB/WAL block record, the Praos chain-dep state, the EpochAccumulator transition (DC-EPOCH-19), and the reduced-UTxO-checkpoint advancement (DC-EPOCH-11). The WAL tail is the single admission authority: recovery drops every block above the WAL-tail slot and reconciles the ChainDB tip TO it, so the authoritative selected-chain prefix is DEFINED as the WAL tail. The chain-dep, the reduced checkpoint, and the accumulator are derived stores -- each a pure function of that prefix. (a) IN-FLIGHT LAG ALLOWED: a derived store may momentarily lag the WAL tail (a torn write, a lazy advance cadence) during live admission, gated by DC-SYNC-01 (the tip is not advanced before the block bytes + WAL record are durable). (b) RECOVERY REMATERIALIZES: on any restart or rollback, every lagging derived store is rebuilt up to the WAL tail by folding its transition over the canonical durable blocks (the accumulator folds apply_selected_block, the reduced checkpoint folds reduced_block_delta, the ledger/chain-dep fold the trusted replay) -- never an ad hoc inverse mutation. (c) FAIL-CLOSED READINESS: validation and forging do not resume until a readiness gate confirms each derived store's last-advanced slot equals the WAL tail (Lagging / Ahead / SeedMismatch / Unsealed are terminal). The only RESUMED state is "all four at the WAL tail"; a mixed prefix (ChainDB at N, accumulator at N-1, checkpoint at N-2) is caught and rematerialized, never run on.